Definitely sounds like asterinas. Most types don't eat zoas but may just cover them up or irritate them. My tank is infested with them as well but they don't actually bother anything. I will take a filter sock and clamp it to my sump, start a siphon and vacuum them up in the morning when they are on the glass and easier to get to. Even with that there are still tons.
Harlequin shrimp eat just starfish so they will clear them out. People who own harlequins will keep chocolate chip starfish in the sump to cut up and feed the harlequins after asterinas are gone so they don't starve.