Blue/green discoloration on plaster
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This summer I noticed that the plaster walls on my spa have turned from their original grey to a bright blue/green. Brushing with a nylon brush did nothing, nor did a wire brush. The only thing that seems to make any difference is a pumice stone.

The spa is 1,300 gallons - concrete and plaster, and 5 years old. It's a salt water system, with an Intellichlor IC20 and a 5 yr old pentair heater..

In the past, when I took the time to monitor the chemistry, it was generally stable, and I added about 1 cup of HCl each week to bring down the pH. I admit that lately, I haven't been as diligent about monitoring - just add the acid each week. No problems with algae (that I can see). Just the blue/green walls.

I recently took a water sample into my local pool chemical supply store, and they detected small amount of copper (0.3ppm). I added some "Metal Free" to the water to hopefully remove the Cu.

Question is: once I remove the Cu from the water, should I expect that I can simply brush the walls to get them back to their original plaster color? Or should I expect that an acid wash is required?

Thanks for any advice.

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