Mushrooms. Had some form of red smooth one that seemed to do fine. Then gone or so I thought. Found one in the lower light part of the tang below a hammer. Hammer grew, stung the mushroom that was coming back, now gone again. No idea how to keep those. Had a frilly green set, clowns thought it was a home and wanted to do some remodeling, and more or less killed it. No idea how but those failed other than the clowns doing whatever they do.
Zoa's are also difficult. Could be light, could be placement, could be some fish eating them - matted file fish comes to mind.