Some infection. If one has a respiratory infection in the nose, sinuses, throat, or the lungs, the damaged cells lining the inside of those areas will ooze some mucus, which is clear but can have some color. If lots of white blood cells, plus dead/dying germs and respiratory lining cells, get mixed in with mucus, one has phlegm. If one drinks adequate water, both phlegm & mucus should get more and looser.
Irritation. Something is irritating the back of your throat (pharynx) or your airways. .. Could be infection (pharyngitis, bronchitis, pneumonia), could be inflammation (post-nasal drip, reflux, smoking, chemical exposure). ..