If you can, take the rock out and kill it. Use something that will murder that SOB before it can spread like the plague. burn it, bleach it, smash it, napalm it, do something! if you cant get the rock out then try mixing some kalk and squirting it on it in the tank with a syringe but be careful because kalk paste will burn corals if it comes in contact.
Good Luck and be vigilant. If you let 1 in the tank then before you know it you'll have dozens and be trying to find something that eats it or boiling your rock.