Depending on the color and condition of your pool’s plaster surface, light copper staining can be hard to notice. Iron tends to make darker, more noticeable stains. Then, when you add AA to the water, the iron is lifted away from the surface but the copper stains are made much, much darker. So what you likely had was a mixed metal scaling/staining problem and the AA only got rid of half the problem.
You need to track down why/how the copper got into your pool. It could be the heaters copper heat exchanging eroded over time from poor chemical maintenance and/or the tabs that were being put in that tablet feeder had copper in them. A lot of retail trichlor tablets sold nowadays have copper added to them in small amounts to act as an algaecide. Clorox “extraBlue” is only one such product, many others add it too. If you don’t scrutinize the chemical ingredient list, it’s easy to overlook. Tabs are especially bad with a heater as the they create an acidic stew of highly chlorinated low pH water that sits inside the feeder. The check valve before it is supposed to “protect”’the heater from back flowing chemicals but the seals on those can easily fail and not do a 100% job of keeping the chemicals out.
