Tan Stain In Plaster Pool

[maker]

Ok, I ordered some absorbic acid, but I am still not sure if it will work. What is the best sequestering agent and I will order it as well. Here are some pictures. Anything look familiar? The little white circle is a return and the two larger ones are where some circular stones fell in over the winter. I cover my pool with a tarp and use the stones as weight. The wind pulled in some stones unfortunately.

I would definitely appreciate your help mbar. I don't want to drain!