Well, this has just become a persistent problem I can't seem to solve. On Saturday I took the advice here and hand-vacuumed every inch of the pool floor that had a little faint green/dirt/debris on it. I spent an hour going step by step sucking up everything. Sunday morning it was faint green/dirt/debris-free, save for one, 1-inch spot and I kept the FC up around 11-12. But then this morning there were a good dozen small spots of faint green on the floor. Ugh.
It's so bizarre, the chemistry all looks good. I haven't let the FC drop below 8 in over a week, and it's usually been at 10+. The CC level has basically been zero for a week. Every OCLT I've done has seen a loss of no more than 1 ppm, sometimes less. The water itself looks sparkling clear, aside from those darn little spots of what appear to be algae on the floor. I've brushed and vacuumed repeatedly.
I guess at this point I'll just deal with it daily for another 2-3 weeks. The season in Connecticut ends before Labor Day for all practical purposes, so maybe I'll SLAM the first week of September and see what happens. Would like to fix this before I close the pool for the season, obviously!
Thanks to all who have given advice.