That is Aptasia, a pest anemone that will reproduce quickly, and can sting fish and coral. You def want to remove it. If you can remove the rock, burn it with a lighter. If you can't remove the rock, there are many approaches. Inject with lemon juice, inject with boiling water, inject with product called Aptasia x. If not too large of an anemone, I've personally had luck with Peppermint shrimp, they will eat it. Sometimes shrimp work and sometimes they dont, think it has to do with where the shrimp are collected.
Whichever way you chose, you need to kill it!