Looks like Aiptasia. It's considered a pest anemone, as it can sting and irritate corals, even killing them in some cases.
Lots of ways to make it go away - kalkwasser as a paste, electrocution with a majano-wand, peppermint shrimp, copperband butterflies, matted filefish . . . Do some research to determine which might work best for you.
~Bruce