Unfortunately, you can't clean pearls in an ultrasonic jewelry cleaner. Not that it won't work - to the contrary, it will work all too well. Pearls are a very soft, organic gem. As such, their outer coating and luster can be removed with ultrasonics.
What you can do, is use a gentle cleaner made for pearls. And we have one. It comes in its own container, with an integrated cleaning basket.
Use is simplicity itself: just put your pearls in, and give the basket a few twirls. No machine or electricity needed!