Quick update - found on another forum a suggestion to bathe him to see if this loosens the skin. Did this (in 1/2" dechlorinated water) and the visible skin appeared to come off, not sure if there is still some left in his mouth though. He then started barking (first time we've heard him do that), assume it was because he was stressed - any thoughts? Put him back in the tank for now, he seems to still have energy, so hoping it's nothing too bad??