Place potatoes in a medium saucepan, and cover with water; bring to a boil. Reduce heat, and simmer for 8 minutes. Add chopped beet greens; cook 2 minutes or until tender. Drain.
Combine milk and the next 5 ingredients (milk through egg whites) in a large bowl, and stir with a whisk. Stir in potato mixture. Melt butter in a medium nonstick skillet coated with cooking spray over medium heat. Pour the egg mixture into skillet; cook 15 minutes or until top is just set. Sprinkle with mozzarella.
Wrap handle of skillet with foil; broil 5 minutes or until golden brown. Garnish with beet green leaves, if desired.