1. what kind of light are they under? That can some times have an issue, move them in to an area with less light for a while
2. how much flow are they in where they are sitting? I found some zoas dont like direct flow, move them in to less flow
3. this is the big one, look for pest like spiders, snails and nudies. all of them can be a hazard and will infest your tank in left unchecked
in my zoa frag tank I have had all including zoa pox with is a good sign that the zoa is really stressed out. I have tried to lower the amount of light but when there in this state you just have to do what you can.
a a few pics will be good for you to get a good opinion.