Bring a large pot of lightly salted water to a boil. Cook elbow macaroni in the boiling water, stirring occasionally, until tender yet firm to the bite, about 8 minutes.

While macaroni is cooking, melt 1 tablespoon butter in a large skillet over medium heat. Add shrimp; sprinkle with Creole seasoning. Cook, stirring continually, for 4 minutes. Add garlic and cook until shrimp is pink, about 2 minutes. Turn off heat.

Drain macaroni and rinse under cold water to stop the cooking; set aside. Return the pasta pot to the stove and melt remaining 3 tablespoons butter over medium heat. Add flour and whisk until dissolved, about 2 minutes. Add half-and-half; heat for 3 minutes.

Whisk in cheeses until melted, about 5 minutes. Add mustard; cook for 2 minutes. Turn off heat.

Add reserved macaroni and shrimp. Stir until heated through. Serve immediately.