Those are ball or glass anemones. They are not a huge problem for most people but they can colonize and become a pest for sure. Quickest solution is to agitate them so that they retract into the hole in the rock and then super glue over the hole so that they can't open to feed and possibly spread. Peppermint shrimp like them as well but may turn to eating some of your zooanthids and other polyp type corals if they run out of pests to feed on.
My ocean. 90g (yup, won it!), 40g, 28g, & 10g Systems PADI Advanced Open Water Tank Thread:
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