Preheat oven to 375°F.Boil potatoes and garlic in a large pot until tender. Drain and return to pot. Mash potatoes and blend in the cream cheese and butter, let slightly cool. Add the egg, then stir in the milk. Evenly pour half of mixture into prepared pan. Cut three-quarters of the mozzarella cheese into small cubes and layer over top the potatoes in the pan. Spread the remaining potatoes into the pan. Bake 25 minutes, remove from oven.