Anemonia sp. usually considered a pest in reef tanks. Simply pry the critter's foot with your fingernail or something sharp ( don't tear it) off the rock and flush.
It look like aptasia, if you pull the rock out and burn it, do so. If not you will have to take a seringe and kalkwasser to get rid of it.
One is not a real problem, the problem is they can spred like wild fire and take over a reef tank. With the stingers and all could also damage other corals. If you have too many now, a copper-banded butterfly fish eats them as well.
Good luck and keep us updated :wink:
