What is this white powder on a pcb
Most likely the contamination is a combination of sticky solder flux and outgassing of battery into white particulate solids around 2-Wire Serial EEPROM and other circuits.

Clean with liberal amounts of isopropyl alcohol, toothbrush, rinse and forced air dry for many hours.

Then beware of battery overcharging.

just to be clear ... flux residue + battery vapour salt condensate + humidity = conductive leakage resistance and capacitance with loss tangent.

Silicone spray is a good choice for protection but only if the added pF of dielectric does not add excess latency to high slew rate logic or excess pF to RF tuned circuits.  Silicone RTV is another option or Mylar/Kapton on battery vent interface,  as a deflector or isolation layer not an airtight battery seal.