The BLACK spots of the picture are actually debris. It's loose on the bottom. The polaris hasn't been used lately because it was just getting too slimy. It's the REDISH BROWN stuff I'm worried about.

Today, on a lark, I tried using my GRILL BRUSH (brass bristles) on them and they WILL brush down. It's orange underneath. I'm going to go out and buy a stiffer bristle brush, and I'm also heating my pool now so I can get into it and scrub harder. I just can't get any leverage on it from the deck..