That looks like a majano anemone. They are a glass anemone and are considered pests due to how fast they can reproduce. Don't pull or scrape at it, any fragment that comes off will turn into a new anemone! Products like Joe's Juice and Aiptasia-X (which is my preference) work well. Others have had success with covering them with kalk paste or injecting them with boiling water or lemon juice.