The white marks are soluble salts which occur naturally in bricks and also the sand and cement of the mortar. These salts migrate to the surface when the brickwork is saturated in a process called efflorescence. It can only occur following saturation or prolonged rainfall and frequently the cause is partially built brickwork being left uncovered overnight.
A building is at its most vulnerable during the early part of construction and will tend to form its own protection when the roof and downpipes are fitted. If efflorescence does occur, brush off as much of the salt as possible when the brickwork is dry and then allow rainfall to wash off the remainder.