I'd bet a dollar to a dime you have a very small pinhole coolant leak in your heater core putting out a spray that's drying to a powder. About the only way that I know to find the leak is to take the heater core out, plug one side (like a piece of heater hose clamped with a dowel rod clamped in the hose end), submerge it and put some very low air pressure on the other side, looking for bubbles. Again, if mine I'd probably plug one side, put a longer hose on the other side clamped tight and rig up a sports ball hand pump w/ needle into the longer hose. They only put out a few psig, so you'd have to work hard to pop the core. You'll probably find a leak in the finned area, which will make repair difficult to impossible. Galvanizing is just a zinc coating, and while it may oxidize in a wet environment it's not going to give you the dandruff flakes you've described. Like Dave says, the guys at VA know AC systems and are friendly.