Preheat the oven to 375 degrees and lightly grease a baking dish. Cook the macaroni in a large pot of boiling water until just tender, then drain and set aside.

Place the spinach and 1/2 cup of the milk in a blender. Blend until completely smooth and set the green mixture aside.

In a saucepan, melt the butter over medium heat. Whisk in the flour and cook for one minute until it bubbles. Slowly whisk in the remaining 1 1/2 cups milk and continue whisking until the sauce thickens.

Remove the saucepan from the heat. Stir in the cheddar cheese, garlic powder, salt, and black pepper until the cheese melts. Add the spinach puree and stir until the sauce turns evenly green.

Combine the cooked macaroni with the green cheese sauce and pour the mixture into the prepared baking dish. In a small bowl, toss the panko breadcrumbs with the olive oil, then sprinkle evenly over the top.

Bake for 20 minutes until the breadcrumbs turn golden and the edges bubble. Let the dish rest for 5 minutes before serving.