This is a fairly common scenario with wild collected yumas. They just don't handle stress well. After they are acclimated to an aquarium for a couple of months, they tend to have more staying power. Captive propagated yumas are the best, but hard to come by. I hate to say this, but usually when one melts off a rock, they all do. I have not found a way to stop it, perhaps someone else might have some suggestions.