Place macaroni, 1 cup cheddar cheese, American cheese, cream cheese and Parmesan into a 4-qt slow cooker.
Stir in milk, evaporated milk, Dijon, paprika and onion powder, making sure the macaroni is submerged as much as possible; season with salt and pepper, to taste.
Cover and cook on low heat for 90 minutes to 2 hours and 30 minutes, stirring every hour until tender and creamy.* Uncover and stir in remaining 1 cup cheddar cheese until melted, about 2-3 minutes. If the mixture is too thick, add more milk as needed.
Serve immediately, garnished with chives, if desired.
Notes
*A brand new 4-qt Crock-Pot was used for multiple recipe testing. Results and time can vary by brand, size and longevity of your slow cooker.