Spray the crock of a slow cooker with cooking spray. Spread hash browns to cover the bottom of the slow cooker crock. Whisk eggs, milk, mustard, salt, and black pepper together in a bowl. Heat a large skillet over medium-high heat. Cook and stir sausage in the hot skillet until browned and crumbly, 5 to 7 minutes; drain and discard grease. Spread sausage over hash browns and spread Cheddar cheese over sausage. Pour egg mixture over cheese. Cook on Low for 6 to 8 hours.