aiptasia. a pest anemone that propogates pretty fast.
to get rid of it you can get some pepermint shrimp (some eat it, some dont) or inject it with a syringe full of joe's juice, lemmon juice, boiling water, or Kalk paste.
i had a few in my tank that came off some live rock and i jsut covered it up with some holdfast epoxy because if once it decays and breaks down, the little pieces can actualy form more.