Why is there white powdery stuff on the walls?

The short answer

[Efflorescence](https://slowmorocco.com/glossary#efflorescence) — mineral salts drawn to the surface by moisture migrating through porous earth walls. The wall is functioning as a wick.

Groundwater carries dissolved minerals — mostly calcium sulfate and sodium chloride — upward through pisé and lime plaster by capillary action. When the water evaporates at the surface, the minerals stay behind.

Modern cement plaster makes it worse. Cement is not breathable — it traps moisture behind the surface until the plaster cracks or pops off. Traditional lime plaster lets the wall breathe, sacrificing appearance for structural health.

There is no permanent fix in the medina. The moisture will return because the ground is wet and the walls are earth. The salt bloom is a symptom of buildings that are still breathing.