A white powder that bleeds trough the cement, grout, bricks and concrete walls and floors. That white residue is called efflorescence. it is the lime and calcium that gets brought to the surface by way or evaporation as the stones dry the water rises to the top and so is the white powder from the cement within the paving stones and retaining walls.
The solution to remove it isn’t many, there are some products that could remove it but at the same time you could damage the finish on the stones. Best to let a professional decide how to and to what products to use.