Cook the macaroni according to package instructions until al dente, then drain.
In a large pot, melt the butter over medium heat and stir in the flour to create a roux.
Gradually whisk in the milk and heavy cream, stirring continuously until thickened.
Add the garlic powder, salt, and pepper, followed by the cheddar and Gruyère cheese, stirring until melted and smooth.
Fold in the cooked macaroni, shrimp, and crab meat gently to combine.
Transfer the mixture to a greased baking dish and smooth the top with a spatula.
Optional: sprinkle extra cheese on top for a cheesy crust, then bake at 350°F (175°C) for 25-30 minutes until bubbly and golden.
Let it cool for a few minutes before serving, allowing the sauce to thicken slightly.