Lightly coat the inside of a slow cooker with nonstick spray. Add the cooked rice, broccoli florets, cream of chicken soup, milk, garlic powder, salt, and black pepper to the slow cooker.
Stir the mixture until the rice and broccoli are evenly coated with the soup and milk.
Sprinkle 1 1/2 cups of the shredded cheddar cheese over the mixture and stir again to distribute the cheese.
Cover and cook on low for 3 hours, stirring once halfway through if possible.
Turn off the heat. Stir in the remaining 1/2 cup of cheddar cheese until it melts into the casserole.
Switch the slow cooker to the warm setting until ready to serve.