Crusty or discharging, runny eyes are often caused by conjunctivitis, or by some respiratory-related infections. Wounds to the eye itself can include puncture wounds, where the eyeball has been damaged by sharp objects, or even a piece of hay.
In either event, veterinary attention is needed; antibiotic eye ointment is often highly effective. If you suspect a wound to the eye, seek help as soon as possible, as early treatment can save the mouse’s eye. Cataracts are sometimes seen in older mice – the front of the eye becomes dull and cloudy.
Sometimes hereditary cataracts are seen in mice of only a few weeks old. There is no practical treatment. Blindness or poor eyesight is not such a problem for mice as for humans, because their eyesight is relatively poor at the best of times.
They can easily adapt to use their other, superior senses more. For blind mice, try to keep the cage furniture (nestbox, toys etc) in the same places, so that the mouse can find its way around easily.