I can tell you what that looks like to me cause I am fighting it right now. That appears to be aiptaisa, a type of pest anemone. There are plenty of products that are designed to kill it but I hear Joe's Juice works well. Don't pull it up because it will break and spread even more you need to use a chemical to kill it. Also, if or once your tank cycles Peppermint Shrimp may eat it.