When you first arrive at the Zora domain in Tears of the Kingdom ,you will find a lot of the city is covered in a thick sludge. The main statue in the middle of the domain is covered in a sludge too and you will pick up a quest that asks you to remove the sludge.
The video guide below will show you exactly how to do this but the quick version is that you will need to get splash fruit. Draw your bow and press Up on the D pad to bring up all of the items in your inventory. Find the splash fruit and use it. When the splash fruit is on the arrow, shoot it at the statue to clear the sludge off.
If you do not have any splash fruit, you will need to go search the mountainside near the Zora domain and you will find plenty of them growing on plants all over the place.
It will only take a single arrow to clear the sludge off the statue. Once this has been done, it will trigger a cut scene and you can then progress to start the main Clues to the Sky quest for the Zora domain which is a LOT of fun to try and work out the riddles for.