Personally, I've only ever heard of amphipods eating zoanthids if the flesh is dead or dying. I've actually seen it on dying polyps in one of my tanks. I've never seen them then go and eat healthy colonies.
I have heard of it a few times. here is a thread about it https://forums.saltwaterfish.com/t/336801/amphipods-eating-my-zoanthids
last week I noticed something eating my wammin watermellons. I dipped them and they seem ok now. Now I noticed some of my organisms are closed up and there are a couple pods crawling around them. Not sure if they are amphipods though.
I think that's just anecdotal (based on the posts in the thread), and likely just sick coral to begin with. I'm sure someone else with chime in to clear it up, but I don't think there are specifically amphipods that go after healthy coral.
I don't have a pic right now, but i have had problems over the last two years on pods eating my HEALTHY zoas. Now i keep a sixline wrasse in both of my tanks to keep the pods down to a controllable level.
thats the exact problem i had.They ate the colony of zoos in my avatar in little under a month.They are NOT reef safe.Once they grow to a large enough size that start eating anything they want.Contrary to everyones beliefs i have witnessed this first hand.