A pale patch on a brick wall often stands out just as the wall dries after rain. One possible explanation is efflorescence. Water dissolves salts, carries them through the masonry and evaporates at the surface, leaving the salts behind. The deposit tells you something about the path of moisture, but its color alone cannot tell you where the water entered.
Start with a wider photograph
Photograph the whole elevation from the ground, then take a closer picture of the patch. Include the nearest downspout, window sill or change in ground level in the wide shot. Note whether the deposit follows a mortar joint, sits below a projecting ledge or forms a band near the bottom of the wall. Keep the date and recent weather with the pictures.
Watch what changes as the wall dries
Photograph the same area again from the same spot after a dry spell. A recurring mark under an overflowing gutter gives a mason a different lead from a light film spread across recently finished brickwork. Neither pattern settles the diagnosis. If the brick faces are flaking or joints are losing mortar, identify those areas separately when arranging brick repairs.
Before agreeing to cleaning, ask what deposit the contractor believes is present, what moisture source they would investigate and where they would test the proposed method. Cleaning the visible face alone cannot stop water carrying fresh salts back to it. The Brick Industry Association technical note on efflorescence explains the conditions needed for efflorescence and how water can reach the wall.
