Spray the inside of the slow cooker bowl with nonstick cooking spray.
In the bowl of the slow cooker, combine the macaroni, cheeses, evaporated milk and skim milk. Stir until combined and the macaroni is almost completely covered by liquid and cheese. Cover and cook on low for 1½ hours.
After 1½ hours, the cheese should be melted and the macaroni should be cooked. Fold and stir in the dry mustard, salt, black pepper, and cayenne pepper. Make sure to stir well to make sure all the macaroni is coated with cheesy goodness.