Ben I am a bit stunned by your comment and a little overwhelmed by all the attached content. I have perused the attachments you provided and am struck that there may not be a "good" solution to this problem, if it is plaster mottling, other than re-plastering. Is my inference correct?

Regarding the pictures the color of the stains on the steps and the swimout (photos 3 & 4) are better depictions of the actual color of the bottom stains, which are influenced by the depth of the water through which they are photographed.

This problem became pronounced about a week after I put a copper algaecide in the pool. It was 1 quart containing 13% of a copper compound. This would have introduced about 3 ppm of copper in a 20k gal. pool I think. I have paused to wonder if that's enough copper to stain/mottle the entire pool to one degree or another.

Thank you for considering my problem.