My 27 yr old whiteface male cockatiel sounded like ha had a cold about a week ago. before this he was very vocal . I noticed he had slimy clear...

Good morning.  Please tell me more about your cockateil's environment.  This time of year temperatures can change back and forth from cold to warm creating stress for some birds.  Mucus from his mouth indicates inflammation or irritation due to several possible factors.  Infection could be a cause.  Also inhaled irritants like smoke, burned food, air fresheners, perfumes.  If he eats table food and ate something too warm or cold that can cause irritation.  

If you have a veterinarian for him, I would make an appointment today and they may be able to swab his crop for culture and microscopic exam then provide the correct medication.  Things you can do at home would be something like covering the cage.  Obtain an automotive clamp light at a hardware store and use a 40 watt bulb.  Put it near the cage so your friend can cuddle up to the warmth.  See if he will take a sugar water with an eye dropper.  This will provide energy and sugar is also antibacterial and antiinflammatory.  Don't stress him by forcing him to take it.  

I look forward to hearing more.