Have a salt generator pool, 30,000 gallon. On well water. pool has been crystal clear all summer. Recently salt generator cell required service which was not noticed until water had turned green. Had pool cover closed during week and did not walk around to pump area to observe indicator light telling me "Hey stupid, I'm not working and I need some service!". Removed cell, did acid clean and placed back in service. Cell did have a lot of what I suspect to be calcium buildup. Muriatic acid bubbled it completely away. Solved cell problem as it functioned properly afterward. To give a shock to water, I added 3 gallon of bleach. Next day there was a vast improvement in clarity of water, but noticed development of black stains on bottom. Took sample to local pool store for test. Test indicated only a need of stabilzer. I asked about indication of metal ions. Was told there was no indication of metal in water. Then I told them there were black stains on vinyl liner. I was asked if these were visible on bottom only. I told them these were only on bottom. I was told that this was black mold that was developing on underside of liner in vermiculite and concrete bottom and that is was showing through liner. If I would maximize chlorine output at salt generator, that this would make stain go away after an unstated period of time. I use to have a sample of the vinyl liner material and I don't remember being thin to point that light could show through it which I think it would have to be that thin to see a dark color in material beneth it.
Does this explanation sound like a possibility of source of black stain in bottom of pool?
Thanks for any input.