Preheat oven to 375°F (190°C). Lightly grease a 9×13-inch baking dish with cooking spray.
In a large mixing bowl, combine cooked chicken, Alfredo sauce, garlic powder, black pepper, and salt. Stir until well mixed. If the sauce feels thick, add 1 tablespoon of milk.
Spread the chicken mixture evenly into the prepared baking dish. Arrange frozen tater tots over the top in neat rows or scatter them evenly.
Sprinkle shredded mozzarella and grated Parmesan evenly over the tater tots.
Place the casserole uncovered in the preheated oven. Bake for 25–30 minutes, or until the top is golden brown and the sauce is bubbling around the edges.
Let the casserole rest for 5 minutes before serving. Garnish with chopped parsley or crumbled bacon if desired.
