There are many causes of eye discharge that range from allergies, infection, dust, injury, and more. The best thing to do is clean the corner of your dog’s eyes regularly with a tissue. You’ll get used to seeing what the discharge looks like and how much accumulates daily. This helps to prevent staining of the fur around the eye and more importantly gives you a baseline so that when you see a discharge that is not what you are used to seeing, you know it’s time for a visit with your veterinarian. Yellow or green looking discharge usually indicates an infection so if you see this talk to your vet as soon as possible. Here’s some good information on eye discharge from Pet WebMD.