The picture is kind of small, but it looks like a vermetid snail. They can be pretty annoying if the population gets out of control.
I would pull it out and use bone cutters, pliers, etc to break off as much of the base as possible. Just be careful as the galaxea's skeleton can be fragile. They'll retract way into the tube so you need to make sure the base of it is thoroughly broken up. I would then put a dab of glue over it just to make sure anything that is left can't survive.