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Check the iron concentration in the well water. Probable oxidation of iron ions to ferric chloride by the hypochlorite. Also check the pH of the water. The pH would be more than 7 in that case.
I concur with Daniel Couton's opinion. Likely oxidation of a metal. Water analysis in this case is necessary. https://doi.org/10.1016/j.ica.2006.03.035
There is a chance that the chlorination process ends up in total chlorine decay and a condition which conducive for an algae bloom..more chemical analysis of the water including breakpoint chlorination analysis is necessary.If the green is due to a condition that permits algae to bloom then use MPC buoys. The real answer lies in a thorough analysis of the raw water and its behaviour upon carefully staged disinfection. Basically the characteristics of the well water chemistry.