Agreed. If they're only on one rock, remove that rock. Do keep an eye out for more though, they are prolific little boogers.
Aiptasia X works well to kill them. It's a painstaking task and you'll probably need to do it several times to get rid of them.
Unfortunately their only natural predators will tend to eat your corals too (ie Raccoon Butterflyfish if your tank is large enough), so the best bet is manual removal.
Jenn