In a medium pot, whisk together milk, flour, and mustard powder until smooth. Heat over medium high heat, whisking constantly, until mixture bubbles and thickens. Reduce heat to low. Add Cheez Whiz, parmesan cheese, and pepper. Stir until cheese are melted. Remove from heat and set aside.
In a deep skillet, cook beef, onions, zucchini, and garlic over medium heat over high heat until meat is no longer pink. Break up any large pieces of beef while it's cooking. Drain off any liquid or fat in pan. Stir in tomatoes and cook 1 more minute.
Add cooked pasta and reserved cheese sauce. Mix well and cook until heated through. Remove skillet from heat. Sprinkle macaroni mixture with shredded cheese. Cover and let stand 5 - 10 minutes before serving.