Sorry, but that is aiptasia. Yes, it is an anenome, but it is a pest. It will spread, it will kill corals and other beneficial inverts. This really is not one you want to keep. I understand that thought pattern of "It's an anenome, if I can keep it alive it means my tank is good", but it's totally, completely false. This particular anenome is tough as nails, spreads very easily, and is nasty. I highly recommend you get rid of them now. Do not cut them, or pull them off, but inject it with boiling water, vinegar, or kalkwasser paste.
I can not tell you how big a headache you'll set yourself up for if you don't not remove it now. My FO tank has a few, and I initially thought, what the heck, there's only a few, and nothing in the tank for them to damage. 6 months later, there were 20, and it took weekly treatments for several months to get rid of them.