I just had a nasty surprise after adding pool shock ("Super Zappit" by PPG Industries, 73% Calcium Hypochlorite, 27% inerts) .... a lot of black patches appeared in a pool that I had thought was under control (except for some light grey patches which appeared after some inattention last October. The black (sometimes very dark red) patches are very resistant to scrubbing - essential immovable. Photo attached. The pool is salt water chlorinated and I have a trickle from chloring tablet chlorinator as back up. Ph is 7.2 and a high chlorine content was indicated by the bright yellow in my test kit even prior to introducing the kit. What do you think is the cause and how can I restore the pool to its former glory?