Agree, looks like a snail with an aptasia anemone (pest!!!) In front of it. Get rid of the aptasia! They multiply like crazy and sting corals and other tank inhabitants. Any pieces that fall off can regrow. On glass you can probably carefully scrape it then suck it out, on rocks I have had excellent success putting a dab of super glue over them as soon as they retract (often into a small hole in the rock). There are a lot of threads on getting rid of aptasia; the trick is catching it early.