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 it little bigger than a quarter, including tentacles. I first noticed it rooted in the sand, but it's been moving all over the tank. The photo shows it on the back wall next to the heater.
Aiptasia. It's a pest anemone that will spread rather quickly. There are a few methods of removal including aiptasia x and spreading kalkwasser paste over its mouth. Just be sure not to scrape it out or it will just grow more.