My white cultured pearls have a "bloom" on them?

by Samantha
(Spain)

Hi,

I was given a beautifull lustrous cultured pearl necklace around 10-15 years ago, they were bought from Hong Kong & they are a fabulous glossy white with beautifull nacre.
I have just taken them out of the box & many of the pearls have what looks like a salty/powdery/calcium bloom on them, I would never have noticed it had I not been running the pearls through my fingers & realised that a few of the pearls felt dusty, I was suprised that it didn't simply brush off. This is predominantly around the area of the pearl which has not come in contact with skin.
I have gently scrapped this off one or two pearls with my nail & this reveals the same glossy white pearl underneath, gritty to the teeth & appears untouched, so it is not a layer of cheap quality nacre peeling off.
What has caused this, & without having them restrung what can I do to clean them?
Kind regards
Sam