Dropping leaves are normal as plants are brought indoors and adjust to indoor light levels. It sounds like this is the situation with your plant's "occasional" leaf drop.

However, Jade plant can get powdery mildew. (Botrytis is also a possibility but less likely.)  Both are fungal diseases, and both are associated with humidity or lack of air circulation.  

Once a leaf is infected, it cannot be cured.  Pick off infected leaves and dispose of them.  If possibly, move the plant to a location where it gets better air circulation and light (for stronger regrowth and faster drying.) Alternatively, move other objects around the plant that may be blocking air movement and light.

 Avoid wetting leaves when you water the plant.  Jade plants need very little water in the winter.  Their soil can be almost bone dry for long stretches with no ill effects.  

Fungicides do not cure infection.  They only work as a preventative.  Once you see the white or gray "fuzz" of a fungal infection, you are seeing the mature state of the fungus and the fungus is sporulating, i.e. sending out spores which will start new infections if they land in the right conditions.   So, the trick is to change the conditions of your jade plant.  First, pull off infected leaves to lessen the amount of spores in the area, and then make sure your plant gets good air circulation and light so that the spores cannot find the right conditions to start growing. 

ECN