You can try rubbing vitamin C on it, or fill a sock with ph minus and rub the stain. If the stain comes off, it is a metal stain. If it doesn't it may be just staining from leaves, or acorns, or algae. If it is the latter, it will come off with the chlorine in the water. You can try rubbing a chlorine puck on it. If the stain doesn't come off with the chlorine, it may be algae under the liner.