Grate the cheeses: Grate the cheddar cheese and, if needed, cut the American cheese into thin strips. This ensures even melting.
Prepare the slow cooker: Spray the inside of your slow cooker with nonstick spray to prevent sticking, then add the macaroni, melted butter, milks, mustard, and salt. Mix in the cheeses until well combined.
Cook the mac and cheese: Cover and cook on low for 1 hour, then stir. Continue cooking for about 1 more hour or until the pasta is tender. Adjust to warm setting to serve.