Coat inside of slow-cooker bowl with nonstick cooking spray.
Combine macaroni, cheddar, Gruyere and American cheeses, milk and evaporated milk in slow-cooker bowl; stir well and cover; cook on HIGH for 3 hours or LOW for 4 hours.
When there is 30 minutes cook time remaining, stir in Worcestershire, mustard, salt and pepper.
Melt butter in a small nonstick skillet over medium-high heat. Stir in panko and cook, stirring often, for 2 minutes or until toasted and golden. Sprinkle over Mac & Cheese and serve immediately.