Question
How do I get rid of Mildew on a Bonsai tree?

Answer
My best guess is that the bonsai tree does not have powdery mildew, but has algae growing on it.  The algae will not kill the tree but looks like a light green fuzz on the surfae the can be rubbed off with the fingers. In either case, I would use a product that has sulfur in it.  Bordeaux Mixture or Wettable Sulfur Powder Fungicide should kill either one.  Mix according to label instructions for an ornamental plant.  Treat outside or over a towel as the Bordeaux Mixture has copper in it, which is blue in color and may stain wood (not permanent) and other surfaces.  You probably need to treat about two or three times to totally get rid of algae.  Let me know if this does not seem to be the problem.  Good Luck.