The powder like substance on your wall can be Efflorescence or calcium carbonate or even the joint compound depending on the wall.
If it's a concrete wall that has a powder like substance it's most often Efflorescence which is the deposit of salts that are left on masonry after water evaporates.
Efflorescence can appear on surfaces like stucco, stone, retaining walls and concrete.
It's not mold or hazardous.
Sometimes drywall or plaster may get a powder substance from moisture that can be the plaster itself or the joint compound between joints as it becomes moist.
In some cases the white powder like substance could indicate mold if it's drywall but most commonly it's nothing serious.
Efflorescence is identified as a white powdery substance that is found on unsealed surfaces such as brick and cement.
It can sometimes appear grey but it is more common to misdiagnose a stain or even mold as efflorescence.
When water comes into contact with your wall, it will dissolve the salt that lives within the wall material.
Once the water evaporates, it moves the salts to the surface and leaves behind a crystalized white powder, otherwise known as efflorescence.
Mold is a type of fungus that can pose serious health risks if left untreated.
It thrives in damp, humid environments and is often seen as a white, powdery growth on walls and other surfaces.