That's an Aiptasia and you should get rid of it, Don't try and pull it out as this will only make it spread all over your tank. If you can remove the rock then you can burn it off with a lighter, if not then you can make a paste solution on Kalk and use a hypodermic and feed the Aiptasia. You can also buy Joes jusice on line or any of the products that can kill them. Also good idea to google "Aiptasia" and read up on them.