The starfish could just be taking advantage of an already injured zoa. If you have a large amphipod population they will eat zoas (mostly at night) and then the starfish will roll up and hit the exposed flesh. At least from my experience. They are both a predator for zoas as far as I'm concerned. Best way to tell is take a look with a flashlight after the lights have been off for a few hours. Solution = shrimp and wrasses.
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