Coat the inside of a 6-quart slow cooker with nonstick cooking spray.
Spread half of the frozen hash browns evenly across the bottom of the slow cooker.
Distribute half of the cooked, drained sausage over the hash brown layer.
Scatter half of the cheddar, half of the mozzarella, and half of the sliced green onions evenly over the sausage.
Repeat the layering process with the remaining hash browns, cooked sausage, cheddar, and mozzarella.
In a large mixing bowl, thoroughly whisk the eggs with the milk, salt, and pepper until the mixture is smooth and the seasonings are evenly incorporated.
Pour the prepared egg mixture gradually and evenly over the layered ingredients, distributing it across the entire surface.
Cover the slow cooker and cook on HIGH for 4 hours or on LOW for 8 hours.
Continue cooking until the eggs are completely set in the center of the casserole.
Spoon the finished breakfast casserole into serving bowls and serve warm.