While you may think Kitty has been crying, gunky red or brown tears under his eyes are actually a sign he's sick. Eye discharge can be a symptom of a number of diseases. He should visit his vet to determine what is giving him eye trouble.
Conjunctivitis, or pink eye, is when the lining of his eyelid becomes infected and inflamed. He'll blink a lot and will have discharge from his eyes. The gunk can get crusty and make it hard for him to open his eyes at all. Pink eye may be all Kitty has or it could be a symptom of another problem, like an upper respiratory infection. His vet will thoroughly examine and clean his eyes, checking for injury or signs of an additional infection. A couple days of treatment of anti-inflammatory ointment usually clears it up.
If Kitty can't produce enough of his own tears, his eyes will become dry, a condition called keratitis. He'll have a mucus discharge and may develop pink eye. You'll have to clean the gunk that builds up around his eyes. Since tears help flush foreign objects out of his eyes, having keratitis means he's at a greater risk of developing secondary infections. He'll need special eye drops from his vet that act like his natural tears. You'll have to put these drops in his eyes for the rest of his life to prevent his eyes becoming too dry.
Upper Respiratory Infection
The most common cause of eye discharge is an upper respiratory infection. Feline herpesvirus or feline calcivirus are usually to blame. Less frequently, chlamydia or another bacteria can be the culprit. If Kitty starts coughing, sneezing, has a runny nose, gunky eyes, or a mild fever it's time for a trip to his vet. Even though it's typically caused by a virus, his vet may prescribe an antibiotic. This will prevent a secondary infection from taking hold while his system fights off the the respiratory infection. Using a vaporizer will allow him to breath easier. You'll need to clean his eyes and nose of any discharge and make sure he's getting enough food and water. He may never completely combat the disease. FHV can go dormant and cause recurring infections for years.
Uveitis and Glaucoma
When the middle of the three linings on Kitty's eyes becomes inflamed, it's called uveitis. It'll cause pain, sensitivity to light, a small pupil and bloody eyes. His vet will give him NSAID pain relievers, like aspirin or ibuprofen, to reduce swelling. If Kitty has preexisting conditions -- such as lymph node problems, feline leukemia, feline herpesvirus or toxoplasmosis -- he'll be at heightened risk for uveitis. Uveitis is the most common reason cats develop glaucoma, or increased pressure in the eye. If he develops glaucoma, the fluid in his eyes won't be able to drain properly. Surgery will be necessary to correct the problem.