Looks like a majano anemone to me. They're a pest anemone that can kill corals if it comes into contact with them. You can spread some kalk paste on it (no powerheads or current in the tank at this time) and leave it on for a half hour or so. This is usually enough to kill the anemone off. Make sure no kalk paste gets on anything else in the tank, or you risk killing it too. If you have a separate tub you could put the piece of rock in to do this, that would be great.