As a former professional airport fire fighter I can perhaps provide
some information on the powder used in the typical dry chemical fire
extinguishers.

The first and probably most common is sodium bicarbonate (baking soda).
It is white in color and is slightly alkaline, and as such will corrode
metal eventually.  This is the typical BC fire extinguisher.

The next is used in professional petroleum fire fighting and is
Potassium bicarbonate (Purple K Powder).  It is slightly purple in
color and is also slightly alkaline.

The last and newest is ammonium bisulfate which is used in the ABC fire
extinguishers.  This is light yellow in color and is also slightly
alkaline.  It also has the property of turning to gum under heat and
surrounding the burning object and smothering it.  This is why it works
(sort of) on wood and paper fires.

All of these respond well to a low to medium pressure (under 50 psi)
air nozzle and are easily dislodged.  I would suggest good ventilation,
as a lung full of baking soda will close off your throat (voice of
experience).  It is no fun wondering when the air will start going to
your lungs again.  I don't think it is poison, but it will sure close
off your throat for a while.

Ammonium bisulfate which has turned to gum responds best to soap and
water and a good scrub brush.  I would hate to do that to a good piano,
but I don't know of any other option.  Hope this helps.

Jim Cook