When a brick/masonry wall is exposed to salt water, or when water is drawn to the surface of the brick or mortar, the natural salts are brought to the surface, where they crystallize into a fine white powder. It is most visibly seen in the edge of the mortar joint directly before touching the brick, but can be visible on any surface.

The reason it is more commonly seen on mortar is because it contains calcium hydroxide, which reacts quickly with carbon dioxide in the air, or with other compounds in the brick. Mortar can also contain sulfates, magnesium hydroxide, and alkalis.

It does not affect the structural integrity of the masonry, and can be easily power washed from the surface, dry brushing, applying a cleaner, or simply left alone to weather away in time.