Preheat oven to 350°F. Remove stems from all mushrooms; set aside 20 mushroom caps. Chop and combine mushroom stems and remaining whole mushrooms.
Melt butter over medium heat. Add chopped mushrooms, salt, pepper and garlic and cook, stirring, for 5 minutes. Place in a bowl; stir in 1/3 cup Parmesan, cream cheese, clams, bread crumbs, Italian seasoning and 1 Tbsp. parsley.
Fill mushroom caps with clam mixture. Pour 3 Tbsp. water into a baking dish and place mushrooms in a single layer; sprinkle with mozzarella and remaining Parmesan.
Bake until tender, 30 minutes. Sprinkle with remaining parsley.