Leafmen are common in areas with a lot of flora, like the Jungle.
There is no conventional way to kill a Leafman. They have a massive amount of Hit Points and regenerate them extremely fast.
In Prospect Station, a Leafman can be seen on top of the Osiris Station.
When crouching repeatedly next to them, they will give off small pink hearts and follow the player for a while.
Leafmen are considered vermin, and "Culling in Progress" signs can be found in closed off areas of Prospect Station.
Leafmen have also been called Leaflings in developer media. However, there's no reference to that name in the game or the game's files.