Place the chicken breasts in the slow cooker and pour the BBQ sauce over them. Cover and cook on low for 6 hours until the chicken is tender.
Shred the chicken right in the slow cooker with two forks and stir it to coat evenly with the sauce.
Add the milk, cream cheese, garlic powder, salt, and pepper. Stir until the cream cheese melts into the mixture.
Mix in the cooked macaroni and 1.5 cups of the cheddar cheese until everything is well combined.
Sprinkle the remaining 0.5 cup of cheddar cheese over the top. Cover and cook on low for 30 more minutes until the cheese melts.
Top with the chopped green onions and serve warm.