Preheat oven to 375°F (190°C) and lightly grease a 9x13-inch baking dish. Grate the Parmesan and mozzarella, mince the garlic, cube the cream cheese, and pat the chicken dry.
Boil salted water and cook the elbow macaroni for 2 minutes less than package directions. Reserve 1 cup pasta water, then drain and set aside without rinsing.
Mix garlic powder, Italian seasoning, salt, and pepper, then rub over the chicken. Sear in olive oil over medium-high heat for 5–7 minutes per side until 165°F, rest 5 minutes, then dice.
Melt butter in a Dutch oven, cook garlic for 30 seconds, whisk in flour for 1 minute, then slowly add cream and milk. Simmer 3–4 minutes until thickened, then whisk in cream cheese, Parmesan, mozzarella, salt, pepper, and nutmeg until silky.
Fold the drained macaroni and diced chicken into the Alfredo sauce until fully coated. Loosen with reserved pasta water a few tablespoons at a time if needed, then taste and adjust seasoning.
Pour into the greased baking dish, top with remaining mozzarella, then toss panko with melted butter and remaining Parmesan and scatter evenly over the top. Bake uncovered 20–25 minutes until bubbling and golden, broiling 1–2 minutes for extra crispness if desired.
Rest the dish for 5–10 minutes, sprinkle with fresh parsley, and serve hot.