Known as Pearl Keeper in Japan, Moe the Clam says it best,
"Spin, spin, spin, and you'll hit, hit, hit the pearl jackpot! Get to know the Pearl Sprite, and she'll give you pearls!"
As Moe says, If you hit PS with a star spin, she will emerge and release a whole bunch of Pearls!
Be wary though, some Pearl Sprites are tricksters, and will toss out spikes along with Pearls.