You can tie the sock onto the skimmer pole and let it sit on the stain. You can put a couple of crushed vitamin C tablets in a thin sock or pantyhose, and tie it on the skimmer pole and let it sit on the stain. If you want to see if the stain is organic, then you can crush a trichlor puck up and do the same. If the stain is llifted by the chlorine tablet, then the stain is organic, and will lighten with high levels of chlorine. If the stain is from metals, then you will need the sequesterer or a stain treatment. Sometimes if the metal stain is new, you can lift it off with just the sequesterer. In my opinion it is a metal stain, because it fits with everything you have posted - and you have a fiberglass pool. You can read back through the metals forum and see that it is very common, and can be taken care of Let me know how the tests go, and I can help you further.