Preheat oven to 350 degrees F (175 degrees C). Lightly grease a 9x13 inch baking dish.
Arrange mushroom caps hollow side up in the baking dish.
In a medium bowl, mix together minced clams, garlic, Parmesan cheese, onion, bread crumbs, green bell pepper, parsley, Italian-style seasoning and black pepper. Slowly stir in approximately 1/2 the butter, enough to make the mixture slightly moist.
Generously fill the mushroom caps with the clam mixture. Sprinkle with mozzarella cheese. Drizzle with remaining butter.
Bake in the preheated oven 30 minutes, or until lightly browned.