Par-cook macaroni noodles by boiling for 6 minutes. Drain & set aside.
Meanwhile, in a small saucepan, melt butter with cheese. Stir until completely melted.
Spray the inside of a 4-QT slow cooker, add sour cream, condensed cheddar cheese soup, milk, salt, pepper, & mustard. Add melted butter & cheese. Mix well.
Stir in cooked macaroni. Cook on low for 2 to 2½ hours, stirring every 45 minutes (equals two times) until the noodles are done & have soaked up a lot of the cheese sauce. Serve as is or as a side dish.