(( For Lights here is my F-List -  https://www.f-list.net/c/dark%20gemini ))

Appearance: Grey appear as a grey-skinned, faceless, hairless, sexless, tall, lanky humanoid alien.

Personality: Grey are introverted, aloof, curious, philosophical, protean in their personality that advances with their age and connection with others. Grey are not inherently hostile they don't tend to be aggressive unless excessively provoked, in which case they kill for the sake of killing, nothing else. Perhaps due to the fact that to kill something denies evolution, and death is anathema to their race.

Relationships: Romance is a learned process for Grey as the custom is not something inherent to their race, it is adopted. Intimacy is possible as a rare odd occurance among them which has lead to some Grey downfall as emotional states overtake their nature and solidifies their forms preventing shifting and an eventual reliance on their partners, sometimes causing the death of them as by exterior means or because they no longer can retain their origional identity and so they decay with their partners.

Mentality: They have hereditary knowledge. If their ancestors have memories of something, their offspring will as well, but if their offspring dies out, the memories die 'with them'. Each Grey is apart of the origional "Grey" all offspring of the origional, are merely descendants of the origional, and possess all the memories and knowledge of their previous ancestors, even if their ancestors and the origional dies, but this does not work in vice-versa, the ancestors cannot attain memories and knowledge of the descendants, this makes them evolutionary in memory, not Hive-minded.
In essence it is possible for each new offspring to be far more wise, charismatic and intelligent than the last. But what is clear is that their learning process is a slow one, and their motives are never exactly clear, only that they seek to learn, and are generally an introverted species, mentally at least.

Reproduction: Reproduction for Grey are unique, they are a race not born of themselves but of others, they are capable of reproducing with either females or males in the 'act' but it appears that they are only capable of replicating themselves through an exterior host, they cannot self-replicate (not asexual). So they require females of another species that can provide the other half as it were. Rendering them sterile to males despite copulation.
Anything mating with a Grey (female species) always produce a Grey, not that of their own race. It is believed the Grey consumes or alters the DNA of the host to form a Grey without flaw in form, to perfectly replicate one of their own.
They are capable of reproducing in any manner so long as a host for their DNA is provided, this allows a Grey to replicate via sperm, egg or otherwise, but they cannot be the host for the same, as their forms inevitably consume whatever is placed inside of them, or ejected from their bodies.
At birth they are produced in the form of malleable egg shapes, this is their first shedding within the passing of three earth-year days, they emerge the size of a dwarf, then after a century (earth years) they shed once more, the bulk of their body falling off and they become frail and lanky looking, now smooth and what is most known to appear as their race, and they are known to live longer than recorded, as if they become ageless after this third stage.
They possess no maternal affection or connection to one another, the child will merely walk when able to do as it's ancestor first did, learning and evolving, and the parent Grey would not see any issue with this nor pay the offspring any mind, where as the other parent who spawned the Grey might.

Homeworld: Grey are from a lightless cold world much akin to a world made of a wasteland geography, there is volanic activity in their world beneath it's surface but the surface itself is effectively "Grey" in the mixture of a super cold and hot atmosphere swirling with smoke, snow and ash in it's unique but uninhabitable atmosphere, it is speculated by scientists as to how these Grey as akin to their homeworld came to pass, but some theorize they come from the core of the planet itself, their planet however is no larger than one of the moons orbiting Uranus in Earth's solar system.
The planet they are from is called The Isolated Wastes, which is a moon surrounded by a gas cluster in white hue at their part of the galaxy, no other planets orbit it's system. Space travel was not known of them, infact their existance wasn't known until after a decade of their planets finding, it is theorized they might not have had a material or recognizeable form, and that their base 'humanoid' form is due to the explorative race that found that planet, this is of course speculation and nothing has been proven.

Origions: It is possible that some manner of organisms had arrived on their homeworld decades before, merging with the planet or upon the planet, having adapted to it and then become apart of it so as to produce the first Grey. Reproduction prior to their retrieval, outward expansion and interaction is unknown.

Languages: Telepathic, Sign Language, any language learned through evolution.

Sustenance: They do not need to sustain themselves with any normal means of food, air, drink or even rest. They are capable of consuming, but it is more of assimilation as an evolutionary process, anything else they reject and it'd harmlessly fall beneath them as if they were liquid to a solid falling through them.

Biology: Grey are a shapeshifter alien that adopts forms and traits of other races to mimic them, an evolutionary alien species. When they've taken on the shape of their new form, they inherently accept that forms DNA and functions, this can be for better or worse, and if they maintain the shape, it could require them to need the sustenance that copied species requires.
They don't appear to possess any bones in their structure and they bleed a quicksilver-like substance, their organs possess no true form other than tubular structure, speculated to be supported by some unknown means, perhaps like plant cells in rigidity. They tend to be a frail species in their normal form but otherwise can become more durable than titanium when assuming the form of something similar by DNA replication.
When they undergo shapeshifting to any manner of race (larger races being more difficult for them to assume) they tend to undergo a means of "Hypermetabolism" which causes them to alter their DNA for replication, this can vary in how long it takes to replicate, depending on the complexity, shape and nature of what it is they are copying. Their minds, reproductive cycle and inherent DNA always remain a constant no matter their form.

Culture: Grey don't possess much of a culture of themselves, they view themselves as reflections of one another, if one Grey views another, they'd identify the other as "I" as if refering to themselves, not to an individual and in fact treat all other Grey as being one in the same regardless of circumstances despite possessing no connection with one another, only with other races do they show any kind of exterior bonding.
Whilst it might be possible for them to possess slaves, there is no record of them doing so, it is either because they view a strong bond in freedom due to their ever curious and malleable natures or because they see no 'purpose' to binding someone to themselves either as an individual or community.
Ultimately the longer Grey tend to linger to one subject they grow to adapt to it and solidify their nature with it for perfect mimicry, this puts them in a state of "Shift Lock" and they risk a high chance of losing their racial identity, and becoming the adopted race.