When using the counter flow chiller in the brewing process, copper is more conductive than stainless and transfers the heat from the wort to the water running in the opposite direction to the flow of the wort.
This rapidly drops the temperature of the wort to pitching temperature. Copper forms a protective copper oxide layer on the surface of the copper when exposed to air.
After the boil, the wort is acidic and low in dissolved oxygen. Because there is low oxygen in the wort when the wort comes into contact with the copper oxide layer, sulphates bind to the surface of the copper to form copper sulphate (green powder) this remains on the surface on the counter flow wort chiller.
When cleaning, hot water and cleaners are used. The hot water and cleaners increase the solubility of the copper sulphate in the cleaning water and is therefore removed from the surface of the copper in the counter flow chiller and left in the cleaning solution. The results of which is elevated levels of copper and sulphate in the cleaning water analysis.
The copper sulphate in the cleaning solution is a good thing as this means it is removed from the surface of the copper in the counter flow wort chiller before the next brew session. If you wish to take this further and reform just the stable copper oxide layer on the counter flow wort chiller then once cleaning is finished recirculate a warm acid-based cleaner like Total San or Star San. The acidity will remove all the copper sulphate and most of the copper oxide, but as soon as the counter flow wort chiller is emptied the stable oxide layer will form protecting the counter flow chiller and the wort.