There are many possible sources of dry white powder in a propane compartment.. First, any aluminum that may be in there turns to white powder if it degrades.. And that is what I suspect. Inspect things like the handle of the tank valve.. ALso look above the tank.. Chalk from the paint on the tank can look like fine powder. Brass, should not turn white and most of the hardware there should be either brass or steel.. The housing for the regulator however, can be the kind of metal that powders if it breaks down or oxidizes So inspect the regulator as well.
Basically INSPECT everything be prepared to buy a new regulator since that is I think the worst case. Every other suspect is less expensive to fix.
Home is where I park it!