Paddy has dry eye which produces sticky grey mucus and sometimes a film of sticky mucus over his eye but I would be v.surprised if all of your's had it - However, it needs a vet to diagnose using a tear test. I am not sure whether conjunctivitis in dogs is contagious (it can be in humans if caused by an eye infection) - think if it's a persistent problem for all I would seek Vets advice as untreated eye problems can be v.problematic later on.
Could you get one dog checked out and then see what vet thinks the problem is ?
Lynne