As water passes through masonry or mortar, crusty, white deposits of salt are leached from the area.
This is referred to as efflorescence/mottling.
Inadequate preparation of the surface. Specifically, when all previous deposits of efflorescence are removed.
A high degree of moistures escapes from behind masonry walls on the exterior.
When it is caused by excessive moisture, block the source of that moisture. Fix any leaks in the roof.
Clear the downspouts and rain gutters. If the masonry contains any cracks, seal them with a caulk that is water-based and premium quality siliconized acrylic or all-acrylic. When the moist air originated within the building, install exhaust fans or another ventilation system.
Vents may be needed especially in the bathrooms, kitchen or laundry room.
Using a wire brush, take off any efflorescence and any loose material. Alternatively, a power washer or power brush may be used.
Thoroughly rinse all debris from the surface. Then apply high-quality solvent-based or water-based sealer or primer for masonry.
Let it dry completely.
Finish by covering the surface with top quality elastomeric wall coating, top quality masonry paint or top quality exterior house paint.