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They will cause harm.. Just wait till it has nothing to feed on then it will turn to your corals. If I were you I would take them out as you see them.Looks like an asterina. I have them in mine and they cause no harm.
+1 on asternia star. Some are harmless. Some will irritate if not eat Zoanthids. Most say that the all white ones are the harmless ones. I pick them off my zoa's when i see them and drop them into my fowl tank. Dog faced puffer loves them as a little snack.