The Grand Pearl is the pearl you have to get in the level Clam Waters, in Aqua Grabber. You get 300 coins for getting it. The Giant Clam has the pearl and to get it, you need to make your way to the bottom of Clam Waters and find the pearl-shaped rock to the right. Put the pearl shaped rock in the Giant Clam's mouth and get the Grand Pearl. After getting the Grand Pearl you find your way out blocked by the Mullet.