David Foster Wallace was a pretty smart guy; for example, one of his unfinished writing projects was his own dictionary! Here’s his entry on:
Mucous: An adjective, not synonymous with the noun mucus. It’s worth noting this not only because the two words are fun but because so many people don’t know the difference. Mucus means the unmentionable stuff itself.
Mucous refers to (1) something that makes or secretes mucus, as in ‘The next morning, his mucous membranes were in rocky shape indeed,’ or (2) something that consists of or resembles mucus, as in ‘The mucous consistency of its eggs kept the diner’s breakfast trade minimal.’