Preheat oven to 375°. Cook macaroni according to package directions for al dente; drain.
Meanwhile, in a 10-in. cast-iron or other ovenproof skillet, cook bacon over medium heat until crisp. Remove to paper towels. In the same pan, add garlic; cook and stir 30 seconds. Stir in flour until blended; gradually whisk in milk and beer. Bring to a boil, stirring constantly; cook until thickened, 2-3 minutes.
Stir in cheeses until blended. Add macaroni, green onions, salt, pepper and half the bacon; stir to combine. Bake, uncovered, until bubbly, 15-20 minutes. Sprinkle with remaining bacon and additional green onions. Serve immediately.