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That is aiptasia! Get rid of it while you can. There are alot of different ways to do it. One way i have heard that works good is to mix up some limewater and inject directly into the anemone with a syringe.
BAD BAD BAD!!! Kill It!!!!! I use Kalk mixed into a thick paste. I then place a couple of drops into its mouth with a syringe. It will close up around the Kalk and shrink into its hole never to be seen again.