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.
That my friend looks like the always annoying aiptasia. since it is a frag tang I would suggest peppermint shrimp, if the frag tank is larger then a Matted filefish. the Kalk past and other chemical products tend to spread the aiptasia if one is not careful