Clean the area in case the umbrella just left a rust stain. There isn’t an easy way to repair it, sometimes adding flame from a torch will even out a small area, but only very carefully and if you know what you are doing. Otherwise, live with it until it’s time for a pool renovation or recoating with ecoFinish. It’s too hard to bring the entire ecoFinish set up out for a small repair, so it’s likely going to have to wait. If you are concerned about leaking, a fiberglass repair may need to be done, there are a few companies that offer diy kits, or perform the repairs.