I can think of a couple of possibilities that it may be.
1. There is a insect called Mealy Bug.
they are little white to grayish white lumps of fluff. At first glance they may appear to look like a moldy spot. If you have these, Insecticidal soap will do the trick.

2 Powdery Mildew.
While powdery mildew usually won't hurt a outside plant, it can play havoc on a jade.
I would move the plant to a dryer location. Stop misting the plant if you have been, and let the soil dry between waterings.
In a bad case, yo may need to treat the plant with a fungicide

One other possibility would be that you have let the plant dry out too much, and the some of the areas between the leaves and branches, may have died. Upon watering the plant again it soaked up the water to these spots and it is starting to decompose there. (just a possibility) If that is the case, you will need to prune all the damaged areas back to healthy tissue.

Hope these ideas help

Mike

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