This is an aiptasia. You do not want them in your tank as they spread very quickly and will sting and kill other corals. There are many methods to get rid off them but do not try to pluck or scrub it off, this will only cause it to spread. I find that injecting the stem with lemon juice works best...or just putting a big blob of it into its mouth. When doing this make sure the current is turned off so the lemon juice can sit in it (i leave mine off for at least 10 minutes). There is also aiptasia x which can be purchased from your LFS and basically works the same way.