Prepare the Slow Cooker: Spray the basin of a 6–8 quart slow cooker with non-stick cooking spray to prevent sticking.
Combine Ingredients: In the slow cooker, mix together diced ham, uncooked macaroni, whole milk, evaporated milk, 3 cups of shredded cheddar cheese, Velveeta cheese, butter, salt, pepper, and ground mustard. Stir to distribute evenly.
Cook on Low: Cover the slow cooker and cook on LOW for 2.5–3 hours, stirring every 30 minutes to prevent the edges from burning and to ensure even cooking.
Add Topping: About 30 minutes before the cook time is up, sprinkle the remaining 1 cup of shredded cheddar cheese over the top. Cover and allow the cheese to melt.
Check for Doneness: The pasta should be tender, and the sauce should be creamy and slightly loose. If needed, cook for an additional 10–15 minutes.
Serve and Enjoy: Turn off the slow cooker and serve immediately. Garnish with fresh parsley or a sprinkle of black pepper for extra flavor.