It's not a metal stain and it doesn't lift using a brush on a pole. I've spent 3 hours at the bottom on scuba literally scrubbing it off a couple of weeks ago. What's left are a few spots that I missed. I could put my tank on again and get down there but the water temp has dropped 12 degrees since the last time which means I will also have to put on a wetsuit, and I don't want to do that.
Will try to get a pic.
Will try to get a pic.