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