A lichen (not to be confused with a lich) is one of the easiest monsters in the game. They are extremely slow and incapable of dealing any actual damage; their main attack is a sticky attack which holds you in one place, but this is trivial considering they are easy to beat. It does, however, prevent you from attacking any target except the lichen, wasting the turn you attempted to attack the other monster (for a minimal impact of two turns as you realise you need to dispose of the lichen first).
Lichen corpses are not considered meat and, unlike most corpses, never rot (but they do become old for the purposes of sacrificing). They can be carried indefinitely as normal comestibles which are always safe to eat, and also used to tame horses. Eat the lichen corpse first however, because it is the same weight as a food ration for a forth of the nutrition.
There is no such thing as a master lichen.
The chamber was of unhewn rock, round, as near as might
be, eighteen or twenty feet across, and gay with rich
variety of fern and moss and lichen. The fern was in
its winter still, or coiling for the spring-tide; but
oss was in abundant life, some feathering, and some
gobleted, and some with fringe of red to it.
Slash'EM Extended has stronger variants of the lichen: aggressive lichen, water lichen, poison lichen, venom lichen, ore lichen, patched lichen, master lichen, grandmaster lichen, sensei lichen and arch-lichen, in increasing order of strength.
The arch-lichen can also be patched in by using the arch-lichen patch (by Nodey) for NetHack 3.4.3.