Not every "white stain" on a wall is efflorescence!
Calcite - calcite, or calcium carbonate, will appear as a thick whitish deposit that usually forms over time. Calcite most commonly forms on brick and block, when excess moisture evaporates and leaves behind a heavy calcium residue.
Lime Run - lime run occurs as a steady release of calcium soaked water that creates a streak. Generally, lime run is treated just like calcite. When it is caught early enough, the normal wash process of NMD 80 (4:1) will be effective enough to remove it.
White Scum - white scum, or calcium silicate, has traditionally come as a result from improper use of new masonry cleaner, reacting with brick and forming insoluble salts. This is created by un-buffered acid combining with silicates and clay.
Integral Water Repellent - integral water repellents or admixtures can sometimes bleed out of the brick or block, leaving behind a "white stain" that has very similar characteristics to efflorescence or white scum; however the removal process is completely different.
As you can see by the pictures below, not all white stains are the same. While some of them appear to be very similar, a simple test with water will help you determine if you have efflorescence or another variety of white stain. Simply spray water on an affected area. If the stain disappears when wet and re-appears when dry, most likely it is efflorescence. If the stain is still visible, most likely it is not efflorescence.