It can be a bit difficult when they are small and not extended out where they are easy to see. But my best guess is aiptasia as well. They can be a serious plague in some tanks, and in others they hardly spread at all. But if I were you, I'd kill it now.
BTW, get use to the idea of having to kill stuff in the tank. Life on the reef can be very cruel and unforgiving. Even some very pretty corals can become a PITA when they meet up with other corals.