I'm not entirely sure but it could be over grooming, or it could have been done by your other glider. I personally would take them to the vet :/
Also, have you thought about putting them on a glider diet with a calcium supplement you feed every day? The pellets aren't the best for keeping them healthy. I have mine on the leadbeaters staple by gliderkids. The glider kids website has a few different recipes, but there's also a lot more out there. They're not expensive and they're pretty easy to make! They include all the protein and calcium they need so you don't have to worry about it at all:)