Preheat the oven to 375°F and lightly grease a small baking dish.
Cook the whole wheat macaroni according to package instructions until al dente; drain and set aside.
Steam or blanch the chopped broccoli until just tender, about 3-4 minutes, then drain.
Season the chicken breast lightly with salt, pepper, and a pinch of garlic powder, then grill or sauté until cooked through. Once cooled slightly, chop into bite-sized pieces.
In a small bowl, combine the low-fat milk, nonfat Greek yogurt, dry mustard, and garlic powder. Stir until well mixed to form a creamy sauce base.
In the baking dish, mix the cooked macaroni, chicken pieces, broccoli, and the creamy sauce. Stir in half of the shredded reduced-fat cheddar cheese, reserving the rest for topping.
Sprinkle the remaining cheese over the top of the mixture.
Bake in the preheated oven for 15-18 minutes, or until the cheese is melted and the dish is heated through.
Remove from oven, let cool slightly, and serve warm.