As part of the main quest in Zelda: Tears of the Kingdom, Link is sent across Hyrule to four major regions: Death Mountain, the Gerudo Desert, the Hebra Mountains, and Zora's Domain. Each region is dealing with a unique issue, and when Link arrives in Zora's Domain, he discovered that the area is blanketed in a mysterious sludge.
The sludge mysteriously rains down from above, coinciding with the advent of the Sky Islands, and is a particular difficulty for the water-reliant Zora people. But before sending the player off to figure out what's causing all of the sludge, Tears of the Kingdom players must clean the Sludge-Covered Statue that sits in the middle of the plaza.
How To Clean the Sludge-Covered Statue in Zora's Domain
After players arrive in Zora's Domain, speak to Yona, the green Zora who is seen talking to Zora named Chroma and Kira in the plaza in front of the statue. The statue recognizes the heroism of Sidon and Link, but it's covered in sludge. Zora have a unique method of cleaning sludge: Splash Fruits. It's only a temporary solution, though, and when Link arrives they're fresh out of stock.
On the way into Zora's Domain players encounter swathes of sludge all over the place, and Splash Fruit can be found all across the hills nearby. Attach one of these Splash Fruit to an Arrow or throw a Splash Fruit (by holding down R and selecting the item in the quick menu) at patches of sludge to make them quickly dissipate.
Like with other sludges throughout the Lanayru region, to clean the Sludge-Covered Statue, just throw or shoot a 
Splash Fruit at it. The sludge will disappear, and the quest can be completed by speaking to Yona again.
Rewards for Clearing Sludge from the Statue
After the sludge has been taken care of, return to the green Zora named Yona to complete the quest. She reveals that she and Prince Sidon are to be wed, and tells Link to meet her at the infirmary. Players are not given a monetary or item reward for completing the Sludge-Covered Statue quest, but it does set off a chain of events that culminate in Link reuniting with Prince Sidon at the top of a nearby waterfall and acquisition of the Zora Armor.
Immediately after completing this quest, head to the infirmary – beneath the throne room in Zora's Domain – and speak to Yona again. She has been working on the 
Zora Armor that allowed players to ascend waterfalls in Breath of the Wild. It serves the same purpose in Tears of the Kingdom, but to complete it, Yona asks for an 
Ancient Arowana fish.
The Legend of Zelda: Tears of the Kingdom is available on the Nintendo Switch.