I have posted on this site before regarding my new pool.
1. after two weeks of operation white plaster started to have full coverage mottled stain. It is light brown to green to black. Worse in some areas..better in others (like top step of pool.)
2. Does not rub off, scrub off, come off under fingernail.
3. Is not moved by chlorine or bleach - yes I got the test kit from Amazon and did follow directions from this site to keep a high chlorine level for three days. Nata.
4. Now, it WILL come off if sprayed with a very light mix of 10% m acid and water. But comes back. Only tested this in the hot tub area.
5. Pool store stumped.
6. Pool guy stumped.
7. Pool builder stumped.
8. No it is not metals. Had feeder water and pool water tested. 0.2 for copper. That is all.
9. Water looks lovely and has always been perfectly clear.
10. I hate this pool and am very upset I paid to have this green monster installed into my yard. What should have been a joy is a constant cause of stress.

My new idea - the gunite sat there curing while we were in full swing of pollen from the trees. If the plaster folks had not washed the gunite off properly - since this pool is under Live Oak trees, is it possible I am seeing minerals leaching from the gunite that can cause stains which are almost impossible to remove?