Preheat the oven to 400°F/180°C, and spray a 9x13" casserole dish with oil.
Whisk the pumpkin, milk, water, salt and pepper in the dish until smooth. Taste it. It should be slightly on the salty side, because the pasta soaks most of it up.
1 can, 2 cups whole milk, 1¼ cups water, 1 teaspoon salt, ½ teaspoon black pepper
Stir in the dry macaroni and half the cheddar, making sure everything is submerged.
10½ ounces dry elbow macaroni, 7 ounces sharp cheddar
Cover tightly with foil and bake for 35 minutes.
Uncover and stir once. Top with the mozzarella and the rest of the cheddar.
7 ounces low-moisture mozzarella
Bake uncovered for 10 to 12 minutes, until melty with a few golden spots. Check at 10. If you want it extra golden on top, broil it for the last 5 minutes.
Let it sit for 10 minutes, then dig in and enjoy!