Tears keep eyes moist and protect them, however many dogs have constant tear tracks under their eyes. Other dogs react to dust or pollen with watery eye secretions. The brownish mucus under the eyes is a normal occurrence, but the tears secreted should be clear. Common tear secretions can be treated at home.

You can gently rinse the dog's eyes with a saline solution from the pharmacy. To do this, drip a few drops of the liquid into the inner corner of the eye and let the dog blink a few times. Then wipe off the excess saline solution with a sterile cotton ball. You can repeat this process up to three times to relieve eye irritation.
When bathing the dog, use a mild dog shampoo and wash the dog's face. Be careful not to get the shampoo in the dog's eyes, as most dogs are very sensitive to it. If some shampoo does get into the dog's eye, it should be rinsed out immediately with plenty of lukewarm water.

If there is yellowish or mucous secretion from the eye, consult a veterinarian. The vet can also give medication for dog allergies, in which eye irritation occurs due to dust or pollen.

Notes:
Greenish secretions indicate an infection that should be treated by a veterinarian.
Flea and insect spray can cause eye irritation in dogs.
The dog should not stick its head out of the window in a moving car, because the wind can cause conjunctivitis and damage the eyes.