That looks like aiptasia. It's bad. Get rid of it. It will spread very quickly and sting your corals, it's a pest. You can smother it with a thick salt paste, a thick kalkwasser paste, or inject it with boiling water or lemon juice. There are also chemicals like Joe's Juice that you can buy at your LFS specifically for aiptasia. Peppermint shrimp will usually eat them too and are a good way to maintain an aiptasia free tank.