Lichen Agaric

This small mushroom is very common and can be recognized most easily by its pleated cap, decurrent gills, a small depression in the center of the cap, and growth on rotten logs covered in green lichen.

The mushroom lichen agaric is the fruitbody of a lichen which you can see growing at the base of the mushroom. When the mushroom/fruitbody rots away, you can still find the primary vegetative part of the organism growing as lichen.