I understand that you are concerned about Johnny Cash's nasal congestion with some green tinged mucous and I'd like to help.
Is the discharge from both sides of the nose?
Nasal congestion is a sign of nasal and pharyngeal irritation.
With nasal irritation we can also see a nasal discharge which can be yellow, white, green, mucoid or even bloody in character. In some cases with long term infections or a mass we can even see bleeding and nasal bone destruction and swelling or changes in nose conformation or around the eye.
A greenish color to the mucous indicates a bacterial component to this, but this may have started for another reason and the bacteria normally found in the nose took advantage and invaded his raw nasal mucosa.
Possible reasons for these symptoms are bacterial, fungal, or viral infections in the upper respiratory system, a foreign body in the nose, a polyp or mass in the nose or a tooth root infection.
Diagnosis of a the problem behind these symptoms can be complicated. We may need to perform radiographs of the nose and sinuses looking for changes in the bones or full sinuses, nasal flushes and scoping to collect culture and biopsy specimens and sometimes blood titers to look for the infection (especially if we are suspicious of a fungal infection).
To check general health I would start with testing for immunosuppressive viruses (feline leukemia and feline immunodeficiency virus) as these allow simple infections to become much worse, as well as a complete blood count and biochemistry profile and T-4.
If those tests come back normal then looking for a tooth root infection, a foreign body or polyp with a nasal scope and radiographs of her tooth roots, nose and sinuses under sedation would be recommended. A biopsy of any abnormal tissue could be diagnostic.
I would also check for a fungal infection by checking blood titers for fungal infections found in your area, aspergillosis tends to cause nasal congestion and infections throughout the US.
If you decide that you cannot or would not run any further diagnostics, and his physical examination is relatively normal then trying an antibiotic such as Clindamycin, Doxycycine or Clavamox could be an option. These antibiotics are good for treating tooth and respiratory tract infections.
Best of luck with your feloow, please let me know if you have any further questions.