OK.
Here's what I think happened: Your pH went in to the 8s, effectively shutting down most of your chlorine's ability to perform. (Somewhere we have a table or formula on that).
That allowed algae to form. The answer is STILL to take your FC up to 20ppm (using 5 121oz jugs of 8.25% bleach, instead of 7).

The only other possibility is that something else unrelated to chlorination is going on and it's not algae. But I don't think so.