Your mouse may have something in it's eye like wood shavings. Maybe try a drop of visine to rinse the eye out. Do it maybe a few times in a day or two to see if anything washes out. Maybe remove some of the shavings while treating so not to get re-infected.
I almost want to suggest a possible poisoning situation? Anyone apposed to the mouse? Could the food been exposed to any contaminates?
I would suggest finding an eye wash solution for small animals if you don't have visine or can't decide the appropriate kind. They may also have an antiseptic eye salve available over the counter. I usually have gotten the eye salve from the vet.
It is a cute mouse and hope it gets better. It hasn't been fighting has it? Good luck!
Scooby, Shaggy the "Dogs", Ms. Thang the "Cat" and introducing Measley Weasle "The Ferret".