Efflorescence

Efflorescence is a naturally occurring substance that appears as a white, powderlike deposit on the face of some concrete, stone, and masonry products. It is caused by soluble salt deposits in either the construction material (bricks, concrete paving stones, mortar, etc.) or the backfill behind the material. Efflorescence often diminishes with time as the soluble salts naturally “sweat out” of the source.

Read more about Efflorescense from Masonry Institute of America

Rust Stains

Rust stains occur due to pieces of steel left on the concrete's surface, which rust and transfer the stain to the concrete. If the surface of the concrete sustains damage from pressure washing or some other form of abrasive wear, a rust stain can occur. Rust stains that occur from the interior of the concrete will often reappear and require periodic removal.

Moss and Algae

Moss and algae can sometimes grow on your paved and retaining wall areas, particularly if they are in shaded areas or covered by trees all of the time.