Spray a 5 or 6-quart slow cooker with cooking spray.
Sprinkle chicken with celery salt, salt and pepper; arrange in slow cooker. Pour 1/3 cup Buffalo sauce over chicken; stir in garlic.
Cover and cook on HIGH heat for about 2 hours or until juice of chicken is clear when thickest part is cut (at least 165°F). Remove chicken to a bowl; using two forks, shred into bite-size pieces. Pour remaining 1/3 cup Buffalo sauce over shredded chicken; set aside.
Add cream cheese and cheddar cheese to the slow cooker; stir to combine. Cover and cook for 15 minutes. Beat with a whisk until cheeses have melted into sauce.
Bring a large pot of salted water to a boil. Cook pasta al dente, according to package directions. Drain well.
Add penne noodles and shredded chicken; stir to combine. Cover and cook about 5 minutes or until heated through. Garnish with blue cheese crumbles, celery and green onions.