Sorry to see the discoloration. I wonder more about your water chemistry and the plaster curing process than stains themselves. You may need to discuss with the builder – as there are a few things that would contribute to this – find out that the correct startup procedure was used for a newly installed finish. I can’t quite tell from the photos what type of finish it is ie diamond brite, pebble, quartz, standard marcite. The startup is important because of the calcium that can leach out of the product and then redeposit on the surface which is what my first guess would be without more information. Another question is how long after the finish was in was salt added to the pool, and was the pool closed with the proper water chemistry balance taking in account the change in temperatures for winter.

The builder may have seen this before and have solutions to remedy – based on what has occurred an acid was to remove calcium/mineral deposits which is my suspicion would be what is needed.