Anyone know what this is?

[Agent Kooper]

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.

 

[Mebbid]

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.

 

[Daniel Fishy]

Perfect place for it, just remove the heater squirt it with lemon juice once it turns like jelly scrape it off and rinse that area really good.