looks like a majano anemone. i would try to get rid of it due to the fact that they can spread rapidly. Some ways to kill them are to inject them with lemon juice or distilled vineger, put some kalk paste over it, or if it is in a tight crack or crevice take a dab of super glue and glue over the majano in to the rock. in a week or so you can pop off the dab of glue and the majano should be gone.