This is the 2nd time ive had a zoa frag shrink and when they did the zoas just vanished after they were so small, i think they broke off just by the water current, i dont know what is happening if maybe a crab or shrimp is eating them or one of my fish, need help all my parameters are good too!!
You might try moving them, I have one morph of zoas that always starts to disappear if you don't give a ton of light. Even next to other thriving zoas it can wither away unless I stick it up top next to my SPS. I also avoid zoa frags with only a few polyps as I don't find myself having great luck with those, they can easily detach and get lost in a high flow reef.