Can't tell from that shot, but if oyur stating it looks like a small Anemone, then its either a Mojano or an Aiptasia, and you need to kill it.
Joes Juice injection
Lemon Juice Injection
Nubibranch
Aiptasia X Injection
Matted Filefish
Copperband Butterflyfish
Peppermint Shrimp (true peppermint shrimp, not camel shrimp)