Combine the shredded cooked chicken, cream cheese, ranch dressing, and buffalo wing sauce in the crockpot. Add shredded cheddar and mozzarella cheeses, garlic powder, and black pepper.

Use a large spatula to stir all ingredients together until well incorporated, ensuring no lumps remain.

Cover the crockpot with its lid, set it to low heat, and cook for approximately 2 to 3 hours, stirring occasionally.

After cooking, stir the dip and garnish with chopped green onions before serving warm with tortilla chips or celery sticks.