Well while I am not positive what is going on with your little guy, I can give some suggestions.
First off, ditch the under-tank heater. Pacs are burrowers that like to go down a bit, usually a couple inches (they then come up to the surface when hungry). When you put a heater under the tank, you are raising the temperature of the moss and then when your pac burrows he cannot escape the heat. Burrowing is also done because it is cooler in the substrate than on land, but putting a heater on the bottom really negates that possibility.
Pacs secrete a whitish film over the top of them when they brumate (hibernate), this locks in water. If this is what is happening to your frog, increase the humidity in your tank so that the substrate is moist. Pacs are from the rainforest so a dry habitat is a bad habitat. What is your humidity at now? What about temperature?
They usually are not that slimy, but sort of bumpy kinda like a toad with a little more slime added on. Sorry that is a horrible way to describe it but that is the best I can come up with.
How many inches of spagnum is in the tank from the bottom up? Also I like to use Bed-A-Beast (which is a brick of coconut fiber that expands with water) or similar product with my pac, it holds water better than moss.
Best of luck with him!