Sand is probably the cause. It got inside and scratched the lense and now it's infected. Get to the vet and have him give you some terramycin. You just put this Ointment on at night before you do to bed. Put it directly on the eye ball. In a few weeks it should clear up. While you're treating your gecko, take all sand out of it's cage. And IMO, you shouldn't put it back in.