Thanks everyone.

Now that I've got some salt in the pool, and the SWG is making chlorine.. I'm seeing less of the buildup.

The concentrated CL on the affected area seems like the next step.

While it doesn't feel organic (I expect organic to feel slippery, not gritty, but I may be wrong about that) the fact that now that I have CL the stain is less, seems to point in that direction.