The friendliest place on the web for anyone with an interest in aquariums or fish keeping!
If you have answers, please help by responding to the unanswered posts.
It's a Majano Anemone, they're considered "pests" because they spread almost as fast as aiptasia but are harder to get rid of, and they will sting nearby inverts and fish
maybe google it for more information but I would get rid of it asap