Many (most?) mac and cheese recipes involve making a mornay sauce, but they rarely call it that. The slightly more informed recipes will actually call it a béchamel sauce and then have you add cheese.
I really hate that when I'm reading a recipe, if it just said "make a mornay sauce with 4 oz cheddar and 4 oz gruyere" you'd cut out 3 steps from the recipe. It's loop unrolling for recipes.