Place bacon in a large skillet and cook over medium-high heat, turning occasionally, until evenly browned, about 10 minutes. Drain on paper towels. Crumble bacon slices into small pieces. Reserve 1/2 cup bacon pieces in a bowl.
Combine remaining bacon, cauliflower, broccoli, mozzarella cheese, and onion in a large bowl.
Mix sugar and milk together in a small bowl until sugar is dissolved. Mix in mayonnaise, Parmesan cheese, and vinegar. Pour dressing over cauliflower mixture. Top with reserved bacon. Chill for 1 hour before serving.