Fly to Mysidia
Once in the overworld fly immediately to your right and you will reach Mysidia. Enter the town, and the elder will greet you and raise the Lunar Whale for you. This ship will allow you to travel to the moon. Get inside and touch the crystal in the middle and it will take you to the moon.
Locate the Cave
There is a plateau that has a ramp downward and will let you enter the set of caves. It is to the upper right of the strange blue crystal towers. Land somewhere on the plateau, and enter the cave below.
Pass through the first cave, heading through the first door you reach. Outside, you will head all the way to the right and enter the next cave. In this cave, go straight ahead. When you exit you will head left, and enter the cave beneath the large blue towers.
When you enter the home of the Lunarians, on the left will be a tile that will restore HP, and to the right is a tile that will restore MP. Go straight ahead, and a sequence will begin which will result in Fusoya joining your party.
Return to Earth
Exit the lair, and return through the caves to the Big Whale. Enter the ship and touch the crystal to return to Earth.