GJB85, welcome to the forum, a good choice! It looks as if it's a metal stain (I suspect iron, but may be wrong on which metal). To test for metal stains, rub some ascorbic acid (pure vitamine C) on the area and see if it disappears. If the ascorbic acid test yields no results, you can try placing a chlorine puck (or small pouch of granular chlorine) on it to see if it's organic. The results of these 2 tests will help the folks here better advise on how to proceed. Looking forward to hearing your results! - Waste