Definitely aiptasia -- a fast-spreading pest anemone. Get rid of it ASAP. Go to the pharmacy and get a syringe (any size is fine). Then inject the aiptasia with lemon juice or boiling water. Alternatively, make a really thick paste out of salt and smother it. Once you get rid of the puffer, buy some peppermint shrimp to make sure the aiptasia doesn't come back.