When you arrive in Zora's Domain, you find some Zora discussing the statue that they have been unable to wash clean.

Wash the Statue

To wash the statue, you can attach a Splash Fruit to an arrow and shoot the statue. Then talk to the Zoras and there will be a cutscene. This begins the quest Sidon of the Zora.

Talk to Yona

Before you go up the mountain, go slightly southwest from the statue of you and Sidon, then go up one of the large curved stairs. It leads up to the area with the throne room. Instead of going into the throne room, go to the area behind it, where you will find Yona. Talk to her to start the quest Restoring the Zora Armor.