Are they getting enough flow? I have found that my multi-colored zoas hate getting tank-dust (no idea what the correct term for this is!) on them.
Moved them to ahigher flow area, and they are doing great. I think it is hard to have too much flow for zoas, although they seem to be very tolerant of numerous conditions. For me, the ones that sread fastest, are the ones with the most flow.
HTH
Stewart