1. Mandarins don't eat amphipods, at least the larger species that look like tiny shrimp. They eat copepods, which are almost invisible to the naked eye. They're small white dots.
2. Your amphipods are most certainly not eating your zoas. If your zoas are closed up and the amphipods are nearby, it is coincidence and nothing more.
As sas said, amphipods eat dead things (dead fish, leftover food, dead corals, etc). They are waaay to small to bring down anything already living. That, and zoas contain one of the most powerful toxins known to mankind. Your amphipods are not eating your zoas.
As a final note, the idea of dealing with a natural problem (too many amphipods) with a natural solution (an animal from the reef to eat them) sounds good in practice, but what we have is about as far from "natural" as you can get it. We dump a chemical salt cocktail into water to make what passes for real seawater, we have a remarkably less diverse amount of life than is in a reef, and the nutrients are out of control in most tanks (compared to the ocean). Adding a fish to correct a problem makes more problems. Once the amphipods are in check, your wrasse begins to starve, or loses a healthy natural food source. Neither is good. Reduce feeding and know that amphipods don't eat healthy, living corals and you'll be just fine.