Preheat the oven to 350°F. Cook the elbow macaroni in a large pot of boiling water until al dente. Drain and set aside.
In a saucepan over medium heat, melt the butter. Whisk in the flour and cook for 1 minute to form a roux.
Slowly whisk in the milk. Continue whisking until the mixture thickens, about 3 to 4 minutes.
Remove the pan from the heat. Stir in 1½ cups shredded mozzarella, the white cheddar, salt, garlic powder, and black pepper until the cheeses melt into a smooth sauce.
Add the drained macaroni to the sauce and stir until evenly coated. Transfer the mixture to an 8x8-inch baking dish.
Sprinkle the remaining ½ cup shredded mozzarella over the top. Use a small ghost-shaped cutter to cut shapes from the mozzarella slices and arrange them on the surface. Slice the black olives and place pieces as eyes on each ghost.
Bake for 20 to 25 minutes until the edges bubble and the top turns lightly golden.