Yes it is possible there are Grey Aliens, but the funny thing is that chance of it being like humans but grey is quite narrow to be almost impossible
We don't know what life would really be like on other worlds as we have not detected any, so using Earth as an example is the only basis we have to formulate evolutionary theories.
Life on Earth could go in any number of directions, and has been evolving since aBiogenesis about 4 billion years ago.
In this enormous time there were billions upon billions of generations, mutating every step of the way, to create the very complex genome humans have today.
At any point in time though, it is worth remembering that a mutation could have evolved in a different way, with equal amounts of success and spawning a completely different evolutionary pathway.
There is a reason why all animals we see have 2 eyes, four limbs, and a spine. This was determined many millions of years ago right back to a series of mutations that (essentially quite randomly) created these features. All animal life since then would then have these traits. That's why we don't see horses with two heads, or rats with three eyes, or cats with only one nostril.
These traits evolved very recently in the history of life, within the range of 400 million years past (within a 4 billion year history) being just the last 7% of the history of life.
So for your aliens to be like us, but just grey, then billions upon billions of mutations must have turned out to be even a little like us except for being grey. Highly unlikely (dare I say, almost impossible). So in order to answer your second question, the environmental requirements that would lead to grey aliens that have a head, 2 eyes, 2 nostrils, would need to be almost precisely Earth (except the minor condition where a grey skin is more advantageous, perhaps they live in underground areas where skin pigmentation is not necessary, or simply grey skin is more sexually attractive).
The truth is we don't know, but all I can say is the chance of meeting an animal with bilateral symmetry, four limbs, two eyes, two nostrils and even an animal with skin, or an animal of similar size to us, would be quite a revelation in itself.