Many syndromes can cause mucus to accumulate around a dog’s eyes. Dried mucus gives the eyes a crusty appearance.
Allergies are a leading cause of crusty eyes in people, but allergies rarely cause eye problems in dogs. Dogs with allergies more frequently suffer skin and ear issues. That said, uncommon is not the same as unheard of. Although it isn’t likely, allergies could be the cause of your dog’s issues.
Other, more likely causes include eye infections, chronically dry eyes (a syndrome called keratoconjunctivitis sicca, or KCS), trauma to or ulcers on the surface of the eyes, and glaucoma.
Several of the problems listed above are potentially serious. Basic tests usually can suss out the cause of crusty eyes. You should have a vet evaluate your dog’s eyes as soon as possible.