White Crust on Soil or Pot
A chalky white crust on the soil surface or creeping up a terracotta pot is mineral salt build-up, not mould, and is a sign of watering and feeding habits worth adjusting.
Common causes
Fertiliser salt build-up
Salts from fertiliser accumulate at the surface as water evaporates. Flush the pot thoroughly with plain water every couple of months and ease off feeding strength.
Hard tap water
Water high in dissolved minerals leaves deposits behind as it evaporates. Switching to filtered, distilled, or rainwater reduces future build-up.
Terracotta wicking
Unglazed clay pots naturally wick mineral-laden water to the outer surface, leaving a white bloom. This is cosmetic and can simply be scrubbed off.