Your dog's eyes should be cleaned on a regular basis, as dogs are susceptible to many of the same eye conditions that are common in humans, including sties, allergies, and infections. If a dog's eyes are healthy, a clear mucus secretion is normal and should be wiped away with a soft-damp cloth. If the secretion is yellowish or bloody, your dog should be taken to the vet right away.
Many light-colored dogs can develop brown tearstains underneath their eyes, usually caused by excess eye fluid that wets the fur around the eyes and allows various yeasts to flourish. To try to avoid these tearstains, keep your dog clean and keep the hair out of his eyes. There are also a number of commercial products that can help to remove any tearstains.