Cook macaroni according to package directions until al dente, then drain and set aside.
In a large pot over medium heat, melt butter and whisk in flour, cooking for 1-2 minutes until golden.
Gradually whisk in milk and continue whisking until sauce thickens, about 5 minutes.
Remove pot from heat and stir in shredded cheddar cheese until completely melted.
Stir in Greek yogurt, garlic powder, salt, and pepper until smooth and well combined.
Add cooked macaroni to the sauce and stir until evenly coated, then serve immediately.