Add 2 gallons of 8.25% bleach, tonight if possible.

Done.

Still at a loss to understand what happened. I have the Taylor K-2006 kit and have been testing daily (sometimes twice a day). I'm pretty certain that I've been doing the FC, CC, and CYA tests properly but I can't think of any other explanation for how algae could have started when I've maintained (or so I thought) pretty high chlorine levels all summer.