Salt does play a role in water chemistry issues, but is not a major player compared to some other issues. Pools in Phoenix Arizona do have a strong tendency to scale due to high calcium hardness of the tap water, high evaporation, and high water temperatures. So calcium scale (which is white to begin with) is likely the problem but usually turns gray, black or turquoise over time due to minerals and dirt sticking to the scale as the others have stated above.
But James may be correct about the possibility of severe gray mottling of the plaster. Mottling and (old) scale can look similar. One way to determine between the two issues is to "feel" the gray areas with your fingers. If it is rough to the touch, and you can scape off the crust with a knife, then that is scale. But if the surface is smooth, then it may mean that it is mottling discoloration of the original plaster job. The integrity (strength) of plaster is probably still good, just discolored.
At this point in time, the plaster job is probably beyond the warranty.
