The password of the treasure chest is 5214
First, you need to find the conch in the circle to trigger the task.
The conch on Twin islands is in the cave below the anchor point and then side missions will be unlocked.
This password is learned through the collection of conch. By collecting conch, you can see the information of 5 songs, namely:
Five Mountains Hide the Little Village, corresponding to the number 5
Two Paths Through the Lonely Forest, corresponding to the number 2
One River Following Down the Middle, corresponding to the number 1
Four Winds Bring the Sound of Joy, corresponding to the number 4
The Clan of Song,Scattered
The last song has nothing to do with the password, so the password for the treasure chest is 5214