OK, so maybe I deserve to take partial responsibility for @TriPolar’s error. Way up thread, from almost a year ago, I had this post:
When I wrote that post, I knew that “mucus” was a noun and “mucous” was an adjective - but I mistakenly wrote that both were nouns.
So, to set the record straight:
Mucus: a noun referring to a slick/gooey material secreted by various parts of an organism.
Mucous: an adjective specifying some kind of connection to mucus. Mucous membranes secrete mucus.
Sorry for fomenting confusion.