zoas slowly disappearing

[JasPR]

100%. Water, light, flow, parameters, or possibly even some other pests. I've seen my share of melting polyps. Think about it. A healthy polyp in your tank... Getting eaten by a pod for some reason... Lets assume this is true. One night pod starts eating it. Would you not see damage? Next night same thing, pod is closed but no visible damage. Next night same thing. Now day 5 (don't know how many days involved here but OP said that the polyps were closed several days), all of a sudden polyp is only half there. Pod must have been really hungry this day I guess because nothing was missing the first few days.

Thats how polyps melt. They stay closed for several days because something isn't right. Something did not get fixed, and the polyp died. They melt and decay from the inside first. The skin tissue stays longer because its tough. Kind of like when people die, the maggots eat your insides first while your skin looks somewhat normal for a few days. (sorry for that graphic) The pic the OP posted above, that polyp is already dead, just the insides are melting through the skin. 1 or 2 days more and nothing will be on that plug. Pods might go eat some of the decaying matter though. For sure. Natures clean up crew.

I have been in the hobby for 40 years, on and off. The powers of observation ( do I beleive you or my lie'n eyes?) sort of response is an understandable and typical beginner response. Triggreef is dead on-- The polyps enter the dying process and the scavangers and opportunists move in for a meal. It LOOKS lik ethey are the attacker or predator, but they are actually the undertaker crew.