That's likely a woodear fungus, genus Auricularia. It's a jelly fungus. More technically, it's a heterobasidomycete. It doesn't cause disease as such, but does decay wood very slightly. Certainly, there is a patch or more of dead wood beneath the fungus.
Don't try this at home, particularly without an expert identification, but woodears are often the mushroom in Chinese hot-and-sour soup.