Why is a white cloud is formed in the hot water from the pot when salt (NaCl) is added?
The white cloud you see in the water is steam bubbles. The grains of salt provide nucleation sites that allow the water to vaporize as they fall through the superheated liquid (so BowlOfRed had it right--although it's steam that is forming, not dissolved gasses coming out of solution). If you raise a pot of water to near boiling and toss in a handful of salt, the water can explode out of the pot due to this effect.
Counter to what you might think, the addition of salt to water actually raises the boiling point by about one half degree Celsius for every 58 grams of salt dissolved per kilogram of water. So the steam is not caused by salty water around the dissolving salt crystals boiling at a lower temperature. The nucleation effect diminishes as the salt diffuses throughout the water.
Note that the effect is not limited to salt. If you toss in something that doesn't dissolve--like sand--you will see the same nucleation effect.