I have these all over my tank and tey didn't seem harmful until my sunset monti wasnt growing where these touched it. they look like the feeding tentacles of something. They are either just one protruding from the hole or two.
the real obvious one is to the right of the chalice. you can also barely see one on the rock below it
the real obvious one is to the right of the chalice. you can also barely see one on the rock below it


