Grease your slow cooker with non-stick spray or use a liner for easy cleanup. Pour in the uncooked 16 oz Elbow Macaroni, 4 cups Whole Milk, 1 cup Heavy Cream, 1 tsp Garlic Powder, Salt and Pepper to taste, 1/2 tsp Ground Mustard, and 1/2 cup Unsalted Butter
Stir in the shredded 4 cups Sharp Cheddar Cheese and the cubed 1 cup Processed American Cheese..
Cover and cook on LOW for 2 to 3 hours. Every slow cooker is different, so start checking the pasta at the 2-hour mark to ensure it stays tender but not mushy. Stir once halfway through to distribute the melting cheese.
Once the pasta is tender, give it one final, vigorous stir to emulsify the sauce. If it looks too thick, splash in a little extra warm milk until you reach your desired creaminess. Serve warm!