Please help identify the source of this stain. I've included a quarter for scale. These little brown circles exist in a few isolated locations in my pool but in particular there are about twenty or so of them near the South West corner in the deep end. They look like little the remnants of little (1/8"-3/16") flat metal circles which stained the pool around their perimeter. However, I could not detect any change with over a minute of ground vitamin C in a sock having been massaged against one. It's almost as if the stain is below the surface of the fiberglass and somehow leeching through. A chlorine tab rubbed against the stain also had no visible impact. Neither has relatively vigorous scrubbing with a nylon brush. The stain in image with the quarter endured all test treatments for reference.
I've only owned the pool for about six months and have no idea how long they've been there. Any ideas? I'm convinced this is a metal stain, and would be willing to try a more vigorous method of cleaning it locally if suggested.
FC:5 CC:0 pH:7.6 TA:90 CH:300 CYA:55 Temp: 89 F