Butter the inside of a 5-quart slow cooker with the softened butter.
Rinse the uncooked pasta well in cold water and shred the cheese while the pasta drains.
Stir together the milk, cream, salt, pepper, mustard, garlic, cayenne pepper, and nutmeg in a measuring cup or bowl. Add the macaroni and cheeses in alternating layers starting with macaroni then cheese then macaroni etc., 3 layers of each. Pour the milk mixture over the layers, making sure pasta is submerged in liquid as much as possible.
Cover and cook on low heat for 1 ½ hours. Stir well and then continue cooking for another hour and a half stirring every 30 minutes. Pasta will be tender and liquid will be thick and creamy when done.
