Look for Furan2. It is a fresh water fish medication used for treatment of several infections/diseases. Do a search for "dipping zoas with Furan" and follow those directions. From the pic I do not believe you have zoa pox. If it is some other kind of infection the Furan2 might help. The black stuff looks like a sponge or bacterial algae to me. If the zoas built up with detritus then the copepods and other critters will pick around them and that can cause them to stay shut for a while from the constant picking.
When you say you lost them in a couple of days - what happened? Did they stay closed and melt away? were they open and pieces were missing?
John
"Critics are men who watch a battle from a high place and then come down and shoot the survivors." Ernest Hemingway