Jade plant leaves can temporarily crystalize if they freeze - this is bad and kills the plant often. They do not enjoy temperatures below about 50°F.
Jade plant leaves can take on a fluffy "crystal covered" appearance when they are sickly due to fungi growing on the leaf surface. This is common with overwatered plants that do not get enough sunlight.
Jade plants have structures on their leaves called hydathodes that can exude mineral rich water that then crystalizes. This can be removed by hosing the plants down.