After completing the main story quest for the Zora, Clues to the Sky and defeating the creature causing the sludge, you will get a quest from Yona called A Token of Friendship. Yona is the wife of the new Zora king. She will tell you to return to the Zora water works to try and find the Zora grieves. Something that is handy to have to complete the Zora armour set.
The Zora waterworks can be found in the main lake, the same place you found it before. Dive into the big whirlpool in the lake and it will take you back down to the Zora waterworks. Once you are down here, follow the tunnel into the main chamber of the waterworks.
You will find that all of the water has been drained from the chamber. This is good as the entrance to the area with the Zora greaves is down a hole in the centre of the room.
Head over to the main tower in the middle of the room and walk around until you find the large hole in the floor. Jump down into this to find a hidden chamber below. There will be three waterfalls in this room. Walk toward the largest of the three waterfalls and walk behind it to find a secret chest. Open the chest and you will find the Zora Greaves inside and the Token of Friendship quest will now be marked as completed.
If you are stuck on where to go here, check out the video guide below. It will show you exactly where to go for this quest to find the hidden area.