In a skillet, combine shredded chicken with garlic powder, paprika, salt, and black pepper. Cook over medium heat until heated through, about 5 minutes.

In a large pot, bring salted water to a boil. Add elbow macaroni and cook according to package instructions until al dente. Drain and set aside.

In the same pot, melt butter over medium heat. Stir in flour and cook for 1 minute. Gradually whisk in milk, stirring constantly until the mixture thickens, about 5 minutes.

Remove from heat and stir in cheddar and mozzarella cheeses until melted and smooth. Add mustard powder if using.

Add the cooked macaroni to the cheese sauce and stir to combine. Transfer to a baking dish and top with the seasoned chicken.

Bake in a preheated oven at 350°F (175°C) for 15-20 minutes until bubbly and golden on top.