If it's not normal shedding, it could be scald/dermatitis if they've spent a long time wet; I've had something similar on the easycares, where they develop completely bald patches with a slight crustyness, but it's not itchy and not scab. I cleared it up by washing with hibiscrub although, just to be sure, i applied crovect a couple of days later just in case there was something else.