hard to tell from the pic but almost looks like bubble algae hanging out in there... emerald crabs eat it or you can try to get it with tweezers just be careful not to pop it..
I have been fighting with it for a while. Debating on just getting rid of the rock, but its a nice piece lol. tweezers and pluck it from its base, Do not pop them or spores spread.
It is bubble algae. You can also pluck it off with your fingers. You can carefully grab it from its base and rock it back and forth gently to get it off without popping it.