Add chicken, salsa, garlic, beans, corn, chili powder, cumin, oregano, ground red pepper, black pepper, and water to slow cooker and stir to combine. Cover and cook for 4 hours on high or 8 hours on low.
To serve, transfer chicken out of slow cooker and shred with two forks. Return to slow cooker and stir. Top with cheddar and green onions.
