river-wear-

You stated that you are not sure why the algae started in the first place. Could it be that the FC reading is the "highest" it is during the day?

I have my pump/SWCG run for 2 hours in the wee morning hours (this is when I add chemicals), and then run an additional 4 hours in mid-late afternoon. Before the pump kicks on in the morning my FC=3-3.5 and after the pump runs for about an hour or so FC raises up to 5ppm.

Could it be that your FC dropped down to ~1-2ppm? If so - that could have been enough to start the algae?