Spray the inside of your slow cooker with non-stick cooking spray. Add uncooked macaroni, shredded cheddar, mozzarella, and cream cheese.
In a bowl, whisk together milk, heavy cream, salt, pepper, and mustard powder. Pour the mixture over the macaroni and cheese. Stir gently to combine.
Cover and cook on low for 2 hours, stirring occasionally to prevent sticking.
Once the macaroni is tender and the cheese is melted, stir in melted butter. Let it sit for 5 minutes before serving for the perfect creamy texture.