It depends on the species. Some will only eat one prey species while others are generalists. Reported prey items include various sponges, anemones, other cnidarians, molluscs, polychaetes, echinoderms, tunicates, & more, but actually the preferred food for most species is unknown.
The best known zoanthid-predators are much different looking. They tend to be fairly short, squat, and thick bodied. Usually they're small in comparison to the zoanthids so they like to settle down & stay for a long lengthy meal. Claws tend to be strong & curved, all the better to hang onto the zoanthids. The images on this page show typical zoanthid-feeders --
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