Combine potatoes, green pepper, onion and meatballs in crockpot and stir well. Combine cream cheese and milk in microwavable bowl and heat on HIGH 1 to 2 minutes or until cheese is softened. Whisk together to blend. Add mustard and garlic pepper and stir.
Pour over ingredients in crockpot BUT DO NOT STIR. Cover and cook on LOW for 6 to 7 hours or until potatoes and meatballs are thoroughly heated. Stir gently and serve.