Re: white mould
If the "white mold" is on the actual surface of a clay pot, it is probably
just fertilizer salts that have been deposited there through evaporation.
If you don't like the looks of it, immerse the pot up to the rim in water
for a few moments, and it should dissolve. A little scrubbing will help.
If there is truly a white mold on the surface of the soil, I would be
concerned. Spray it with a fungicide.
Robin in Tampa