Rock hard stain build up
When more moisture arrives through the substrate it brings more salt deposits to the surface and so on building, a white looking salt stain on the surface.
The Stain Eaters have a cleaning solution that breaks down the salt and removes it from the surface.
If in an external situation, water under pressure can be used. If internal, a hand cleaning technique is safest with appropriate controls around the environment to ensure that other infrastructure is not effected.
Unfortunately, the salts will reform until the source of the moisture is eliminated from behind the wall.
Surface sealers are ineffective because the moisture buildup eventually pushes the sealer off the wall.