The number one source of mucus is the mucosal linings (so-called mucus membranes) of our insides, from our mouths, nose and eyes throughout our lungs, stomach, and gut, reproductive and other organs.

Anyone with a healthy, hydrated body with a functioning immune system has to produce certain amounts of mucus to live healthily. It protects those mucosal tissues throughout the body from dryness and irritation/infection. If weed is increasing your mucus production, it's probably because you get some irritation from it and are having what's called a histamine response, which causes your mucus membranes to secrete more mucus in defense of the irritation. You can take a slight anti-histamine like Benadryl to dry up some of that secretion. You can cough to clear it. Or try taking it in some other form like vaporizing or eating it.

(Milk is indeed a highly muco-genic food, but foods and irritants of all sorts can also be muco-genic.)