PREHEAT oven to 350F. Spray a 13x9-inch baking dish with no stick cooking spray; set aside. Cook macaroni as directed on package using minimum cook time. Drain well.
MEANWHILE, melt butter in 3-quart saucepan on low heat. Whisk in flour; cook and stir until smooth. Gradually stir in milk. Bring to boil, stirring constantly. Boil 1 minute until mixture is hot and bubbly. Add cheese and 1/2 cup of the RedHot Sauce; stir until cheese is melted and mixture is smooth.
ADD cooked macaroni, chicken and celery to cheese sauce; stir gently to coat well.
POUR macaroni mixture into prepared baking dish. Sprinkle with green onions and blue cheese crumbles if desired.
DRIZZLE top with remaining 2 tablespoons RedHot Sauce. Bake, uncovered, 15 to 20 minutes or until bubbly and lightly browned on top. Let stand 5 minutes before serving.