It is caused by the scent glands.

You can give the mouse some anti-inflammatory drugs, put eye drops in the eyes, and then apply chlortetracycline eye ointment. It will disappear after two days. The reason is that it comes into contact with unclean things. It may also be It's the hamster's claws that keep scratching. You should also be careful not to let him wipe it off after you apply it.