When CYA is added to a pool, a higher FC level is needed to prevent algae (but you already knew that). Though you were worried about too high a chlorine level, it would appear that in this pool there is too little chlorine generation to maintain a high enough FC, especially for the CYA level, though I don't know why. Perhaps there was even more CYA added than you think -- a full set of measurements would be helpful. If the FC was very low to begin with, then adding CYA would have made the disinfecting chlorine level too low so algae could develop.

Also, if you use CYA in the indoor pool I had also mentioned that the use of non-chlorine shock might be required since achieving breakpoint with chlorine alone is always difficult in an indoor pool (due to lack of sunlight) and that with CYA it's even harder. If you add chlorine and find that the FC eventually gets stable (doesn't get consumed) but that the CC level is high and doesn't go down, then the use of non-chlorine shock (KMPS) would probably be needed to shock and as ongoing maintenance, though in theory the SWG should be able to shock the pool even with CYA in it.

I'm still betting that the SWG stopped working or was set too low. Either that or the amount of CYA that was added was a lot more than you thought.

Richard