Looks like a good case of the flatworms. Flatworm exit didn't work for me, but it has has given others good results with increased dosage of up to 5-10x. Do this at your own risk and desrepancy. Just be ready with water change water because when they die, they release a toxin. Suck out as many as you can before you dose it and about five minutes after. Then do a 20% water change. Good luck
Are they moving? They could be flatworms. If they are flatworms, I dont think they are responsible for the zoas disappearing. Nudibranches could cause the zoas to disappear, have you seen any slug like things on them? Also, zoas will sometimes just melt away for no reason i can figure out.
Those are flatworms, which are a pest. But they may or may not be the reason for the zoas disappearing. Even if they are not causing your zoas to die, you still need to address the flatworms and I would treat with Flatworm Exit.
Odds are, you do and it won't eat them anyway. Then you have a fish that you might not have included in your stocking plans. Six- line wrasses may eat them, the yellow coris wrasse is better known for eating them, but again, it's not a sure thing, and that fish wouldn't fit in your system. Try the flatworm exit.