That's an aiptasia -- pest anemone. You should kill it ASAP. :(
The best thing I've found is a product called Aiptasia X. It's easy to use and reef safe. And it's EFFECTIVE! Other solutions are to use a salt paste, a kalkwasster paste, inject it with lemon juice or vinegar -- I've tried those too, and those methods didn't work as well.
You can also add some peppermint shrimp and hope that they'll eat it. They usually do.