Here is how I see this "white" quartz plaster turning "gray" problem.
1. The start-up and ongoing water chemistry conditions should always be considered when investigating most plaster problems, and for this case, the OP indicates that the pool and tap water is/was balanced. Therefore, that establishes a fairly reasonable basis to exclude "out-of-balance water" as a probable cause. And graying of the plaster itself, rather than a gray stain or deposit (which still needs to be verified by the OP by sanding a small area), makes it more likely that water chemistry is not the cause of graying. And further, aggressive water does not cause white plaster to turn gray, and especially in unusual patterns and areas.
2. According to the weather almanac, the temperatures in St. George Utah in or around January 5, 2016, was from 40 degrees in the morning and up to 50 degrees in the afternoon. The OP stated that the plastering job was completed in 4 to 5 hours. I believe that the plastering contractor probably added a cement hardening accelerator to be able to finish the plastering job within that time frame. Otherwise, I believe it would have taken more than 8 hours to complete. The OP stated that he heard a crew member indicate that they had run out of "calcium" during the job. That most likely indicates that "calcium chloride" was being added to the plaster mix, and that they didn't bring enough to compensate for the amount that they were adding.
3. The addition of Calcium chloride is well-known for causing the graying of both gray and white cement. It also causes shrinkage cracking when too much is added. It very well could be that the gray areas in this pool are due to excess calcium chloride being added, and the white areas are where NO calcium chloride was added. Calcium chloride affects the hydration (hardening) rates of the various types of cement compounds creating lighter and darker effects. This is why I asked that the plaster contractor should state if calcium chloride was added and how much was added to each batch of plaster material.
4. It is also well documented that late hard troweling also contributes to the darkening or graying of white (and gray) cement products, which could also be an issue with this plaster job and contributed to the graying discoloration.
5. It is well documented that the sprinkling of water while troweling can cause a difference in water-to-cement ratios on a cement surface and create "light and darker" areas on the cement surface. Certainly, other water filling and splashing issues can also affect the cement hydration process and therefore, cause differences in white cement shading or graying.
6. An analysis of how much calcium chloride was added to the plaster mix can be easily performed, and only costs about $100. I believe that step that needs to be taken in order to learn what may have occurred to have caused the graying. Generally, less than 2% calcium chloride should be added to white cement products. Pool plaster contains about 35% to 40% (white) cement and the rest is an aggregate that doesn't get involved with discoloration. Concrete flatwork, such as sidewalks and driveways, and pool plaster are very similar and are applicable to the same discoloring and defective issues due to improper practices.