In my opinion, that is not normal mottling, and I highly doubt that the color will improve and darken over time and become more even and consistent.
There can be several reasons for new dark colored plaster jobs to lighten and become uniformly whitish, or become whitish in streaked, splotchy or mottled patterns.
Generally, if gray or black plaster turns uniformly whiter, then it is probably due to water chemistry issues - perhaps during start-up period (first month), or during the following months. If pH goes above 8.4, (and depending on the TA and CH), a likely uniform whitening (due to scale) may occur. If it is white scale, a simple hand sanding with 100 grit sandpaper will remove the whiteness immediately. Also, a plastic bottle with diluted (50/50) acid and squirted on the plaster surface (under water) will also remove recently scaled plaster on new pools. If the whitish areas are not removed in that manner, then the following issues may be involved.
If the pool is filled with very soft (aggressive) water, that can dissolve and etch cement material off the surface. That will slightly lighten the color of dark plaster jobs. An acid start-up will do the same thing.
Another cause of whitening of gray plaster is due to adding calcium chloride to the plaster mix. And understand that there are usually several batches of plaster during the plastering of a pool. Therefore, one batch of plaster can have more calcium chloride added than in other batches.
Another cause can be from improper troweling procedures, such as adding too much water while troweling and forcing that water into the cement surface. That will skew the water to cement ratio in various or localized areas. When that is the case, it causes a non-uniform whitening, or in other words, it causes streaking, splotchiness, and various forms of mottling.
Lastly, if the plasterer actually use a propane torch to dry and harden the plaster during a cold winter day, that could also cause a light colored mottling effect in those areas. Not a good thing to do!
