I see this all the time... as a masonry contractor/general contractor..
it is what the trade calls "efflorescence" . it appears often after rains .. it is derived from impurities in concrete, mortar, brick, or any other cementous products .... the impurities are gathered by the water and drawn to the surface by the sun and can literally grow like a crystal formation ... it is usually sodium or alkaline based... it will usually taste like salt.. but you probably don't want to taste it..
http://en.wikipedia.org/wiki/Efflorescence
hope this helps