Remove mushroom stems; set caps aside. In a large bowl, combine the clams, bread crumbs, Parmesan cheese, green pepper, onion, garlic, Italian seasoning, parsley & pepper. Stir in 3/4 cup butter. Fill each mushroom cap with about 1 tablespoon clam mixture.
Place in an ungreased 15x10x1 inch baking pan. Sprinkle with mozzarella cheese; drizzle with remaining butter. Bake, uncovered, at 350 for 20-25 minutes or until lightly browned. Serve warm