On Sudeki,In Nassaria's grotto there is a green button, to the left of said green button is 5 blue circles.The button plays Nassaria's song.The 5 circles represent a different note in Nassaria's song.To get 1 of the mermaid pearls,you need to play Nassaria's song on said circles.This is the combination:
(2)
(4) (5)
(1) (3)