How many zoas are we talking about here? The most likely explaination is that the zoas are deteriorating and the pods
are working their way through the decaying tissue. True cases of amphipods attacking and eating healthy zoanthids are extremely rare, as they are detrivores, not carnivores. It is not likely that your going to remove all the pods
from your aquarium, you can't treat them with anything as the corals are invertebrates as well, and you likely can't move the zoas to the sump area as it is likely full of pods
as well. I'd check the zoanthids for decaying polyps/tissue and remove those that are infected.