Prepare the Cheese: Grate the cheddar and American cheeses. If using American cheese slices, cut them into thin strips.
Set Up the Slow Cooker: Spray the crock of a 6 to 8-quart slow cooker with nonstick cooking spray.
Combine Ingredients: Add uncooked macaroni, melted butter, evaporated milk, whole milk, dry mustard, and salt to the slow cooker. Stir to combine.
Add Cheese: Stir in the grated cheddar and American cheeses until evenly distributed.
Cook: Cover and cook on Low for 1 hour. Stir well, then cover and cook for 1 additional hour, or until the pasta is tender.
Adjust Consistency: Switch the slow cooker to warm. If the sauce thickens too much, gradually stir in warm milk to adjust the consistency.
Serve: Season with additional salt and pepper if desired, and serve immediately.