Hydrostatic pressure describes the pressure caused by standing water pushing against an object or surface that blocks it. In the picture here, water that has no where else to go is held in the dirt behind this wall. The water has to get out somehow, and in this case it leaves through evaporation. As the sun draws the water molecules out through the wall, it peels the paint. Garden walls and retaining walls are notorious for having this problem.
Efflorescence
Efflorescence is a white powdery deposit of salt crystals on cementitious wall surfaces. It is unsightly and happens when water behind the wall transports calcium and lime salts to the outer surface. The white powder can be cleaned off, but will return when the wall gets wet again. It can be a visual problem on stucco, cement and brick walls.
