The phrase "milk sub" refers to a substitute or alternative for milk. It could be any other drink or product that is used in place of milk, such as soy milk or almond milk. Full definition
What would you recommend for an almond milk sub — very allergic to it? (thehealthymaven.com)
I mean, mashed potatoes usually require a load of milk or cream and butter — and sorry, almond milk is not a great milk sub in this case! (eluxemagazine.com)