That's a Ball Anemone - a Corallimorph. They're related to soft mushroom corals. They can sometimes multiply rapidly, but they don't usually cause any issues, so a lot of people like to keep them.
That's a Ball Anemone - a Corallimorph. They're related to soft mushroom corals. They can sometimes multiply rapidly, but they don't usually cause any issues, so a lot of people like to keep them.