Cut green onions into three-four. Remove tails from shrimp.
Peel potatoes, wrap in a plastic wrap and heat in a microwave for 7-8 minutes or until tender. Then mash in a bowl. Add milk and butter and mix well until smooth and creamy. Season with salt and pepper.
Preheat the oven to 450 degrees F.
In a lightly buttered casserole/baking dish, spread half of the mashed potatoes, place shrimp and green onions. Then spread the rest of mashed potatoes. Pour the mayonnaise along the side of the casserole/baking dish. Sprinkle mozzarella cheese on top. Bake for 20 minutes.