Yea amphipods are what most refer to being the problem when having issues with coral trouble. The population in my last system was so large with so many refuge areas they were everywhere. They will irritate a coral to the point where it suffers enough for them to take advantage of it and from what I have seen, actually eat the flesh that’s still alive. Might not be healthy flesh but the flesh was for sure not dead and flaking off. I was seeing little ridges and trails of patches where they were eating. I had all sorts of fish that were supposed to eat them but numbers were steady strong. Mandarin fish I have never seen eat amphipods this large either. Only larger wrasse.
Good luck to anyone with this issue. I would recommend keeping detritus out of the system and clean areas where they hide like refuges with rock and refugiums.