In a large bowl, combine the sour cream, cream cheese, milk, ranch dressing mix, garlic powder, onion powder, and black pepper. Mix until smooth and well combined.
Place the halved baby potatoes in the slow cooker. Pour the ranch mixture over the potatoes and stir gently to coat.
Cover the slow cooker and cook on low for 4 to 5 hours or on high for 2 to 3 hours, until the potatoes are tender.
Once cooked, stir in the shredded cheddar cheese until melted and combined.
Serve warm, garnished with chopped green onions.