http://www.mayleen.com/rcentral/mousecare.htm
might have some answers for you
Mice can be allergic to food and beddings.
They can also get skin infections from infection of a single open bite or scratch.
You may need a vet visit and antibiotics to resolve the problem you are seeing.
As its only one out the two mice with the problem something like mites or a fungus seems less likely.
You can try some neosporin ointment on the sore spots to see if they will help them heal up.
Hope this helps you!
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