Were you really keeping your Total Free Chlorine (FC) at 15 ppm the whole time before this algae appeared? Even with 90 ppm CYA, the amount of disinfecting chlorine (HOCl at pH 7.5) would be 0.090 ppm which should be plenty to prevent algae (at least that's what we think, so getting more info from people like yourself is helpful).

The shock level of 23 ppm results in an HOCl level of 0.177 so let us know if this takes care of the algae.

Is the area where the algae occurred a place where there might be poor circulation?

Finally, do you have any idea what your borate level is, given that you use the 3B's which includes using Borax to raise the pH? If you don't know the level, how often and how much Borax do you add?

Richard