After a year or so of storage in ZipLoc bags in the fridge, my home-dried prunes have developed a bit of whitish crust. At first I thought it was mold, but it seems like a crystalline efflorescence, and doesn't smell or taste bad. The prunes seem normal and taste fine.
The prunes, freshly picked from my tree, weren't pre-treated before drying, but after drying were lightly sprayed with a solution of potassium sorbate (mold inhibitor) at the recommended strength.
My current hypothesis is that this is sugar that has migrated out of the fruit and recrystallized on the surface, but I have no idea if this is right.