It’s hard to see, are you talking about the little tentacle at the bottom of the rock? If so, it’s one of three things, either a vermetid snail, spaghetti worm, or a mini brittle star. The only one of those to worry about is the vermetid, and even then unless they’re at plague proportions, I wouldn’t tear the tank down. For the ones that are bothering corals, either completely remove the shell, or clog the end of it with superglue. Some wrasses and bumblebee snails have been said to eat them, but it’s no guarantee, so only get them if you actually want them as tank inhabitants. But based on the picture it’s hard to say exactly what they are. Also, spaghetti worms can also irritate corals, but they’re not to hard to move if they’re in the sand and they’re otherwise beneficial.
