As you go through the underwater maze, stay near the top, then down the right, and across the bottom, until you reach the "gnarly monster" sign. The giant clam with the pearl is just up and left of the cave entrance.
(see related question)
You do nothing. Its one of the items that you'll have to give to Zeus.
Your quest on Mythology Island is to give Zeus the 5 items;the rose, the whisker, the scale, the pearl, and the golden ring. To do this, you have to get the apple (to summon Zeus and then actually get the quest).
Zeus will give you a list of sacred items. These include: A flower from the Garden of Spinx, a golden rign from the Minotaur, a giant pearl from the kingdom of posiden, and a whisker from Cerberus, the Gurdian of the Underworld.
Go left out into the water, where you can swim down to see the Hydra and the Giant Pearl. The bubbles give you plenty of oxygen, and you should go acroos the top, down the right side, and then left across the bottom.
a pomagranete
To give Zeus the five sacred objects on Mythology Island in Poptropica, you need to find and retrieve each one from different locations on the island. The sacred objects include Poseidon's conch shell, the golden vase, the giant pearl, the flower from the Garden of the Sphinx, and the shield from the Minotaur. Once you have all five items, return to Zeus at the top of Mount Olympus to complete the quest.
a pomengranite
you have to get the pots for the guy so he will give you it
You jump on its heads and it will give you the scale.
you go to athena and give it to her
give him ten darchmas
You jump on its heads and it will give you the scale.