The Grays are an alien species that first appeared in "High Strangeness." Not much is known save for that they're hinted with the usual tropes that come with their story. Abduction, hinted genetic experiments which result in hybrid children, though they do appear to be friendly.
The Grays are for the most part at least in "High Strangeness" known for their mystery and abduction. Tree Trunks was one of their recurring subjects, though she considered it an adventure and believed they were one of her many admirers. With her they made several hybrid children.
In Adventure Time: Distant Lands first episode BMO, the Grays are given a bit of a cameo appearance. CGO reveals that Hugo came from the Earth to escape from a war. Its shown that when his ship first hit turbulence the Grays quickly came to his aid and offered his team shelter. Hugo became instantly fascinated by the alien technology and was interested in what CGO called "biohacking". Though the Grays warned him about the dangers of biohacking, to quote CGO "Hugo always gets his way".
This shows that he experimented on himself and his followers DNA with the genetic material of the Gray aliens thus creating Gray Elves. This did not come without consequences, as it apparently left the Grays vulnerable to disease; though its unknown if it was due to human's pathogens or a result of "biohacking" as CGO referred. This caused the death of several Grays. It is possible that Hugo's interference might have led the remaining Grays to actions left in "High Strangeness"
The Grays are all slim, gray humanoids with large, bulbous heads and dark green eyes, very much resembling the traditional gray aliens of pop culture.