Did you try to use a magic eraser? If the stains are only on the steps you might want to try one just to see if the stain is really on the fiberglass or just lying on top of the fiberglass. I also noticed one time when I had some stains and my ph was high, I added muriatic acid by the stain and it disappeared. You may just have to run your pool at a lower ph. You just don't want to go under 7.0. It just seems strange to me that the stains are only on the steps.