what are the water parameters?, how big is the tank, and what are its tank mates? I had a male covered like this once and I moved him to a different tank away from all his tank mates and he came right again, moved him back it started again, in the ned he had his own little palace for the rest of his life, i put it down to stress in the end, it always looked like excess slime coat to me. he was a bit neurotic though, even stressed out when i put a mirror in there.