Black algae and trimming too short..or co2?

I know they share a direct relationship (BBA and co2) with each other; and I assumed it was based on flow and not just co2 enrichment.

My PH and KH show I'm at 40ppm CO2 in the water; my drop checker (I know, I know, I dislike it too...) is a very faint light green....I know my co2 is fine. I blamed the 2 leaves on 3 plants that have BBA on them on a lack of flow in that area. I installed a power head, and didn't aim it right at that area; but did point it just above it to make sure water/co2 circulation was sufficient there.

Now, these plants I did trim a little too short in my opinion, and it caused the original growth to stop, and new side shoots started, which pearl like crazy and have taken off. The leaves I speak of are from the original stem...Could the original stem be dying, and thus causing bba to form on the remaining leaves? Thank you!