Unless I'm mistaken it looks like aiptasia, which is bad. If you do a search on aiptasia, you'll find countless threads. If you touch it with a stick/rod and it recoils into the rock this is another sign its aiptasia.
Looks just like an aiptasia only a little browner. I'd keep an exceptionally close eye on it, as aiptasia and its relatives pack a powerful sting against most slatwater life and can reproduce quickly
Agreed, were that an aiptasia there would be burn marks on all the nearby mushrooms courtesy of the aiptasia's powerful sting. More like a feather duster.