It's an Aquilonastra spp. starfish, beneficial. Often misidentfied as an Asterina spp. Aquilonastra species primary method of reproduction by splitting (fissiparous) is unique. ( Astrina are rather shortlived and only reproduce sexually so are unlikely to maintain viable populations.) Aquilonastra feed on biofilms and algae. In the case of aging biofilms or biofilms that have shifted to pathogen dominate they may provide as essential service to corals helping corals renew their biofilms.
I've also seen them on my zoas. The mucus film on corals ages and can start to favor pathogenic microbes and has to be shed. (And FWIW we can't use color as an indicator of health, in fact, bight colors can indicate a unhealthy coral.) In all likelyhood the Aquilonastra were doing your zoas a favor by removing aged biofilms.