[AaronT]

That is definitely black brush algae. Flourish excel when dosed at the recommended amount on the bottle will hinder any more growth and is safe for shrimp at those levels.

There's more to algae than just dosing, CO2 levels and lighting. Water circulation, water clarity / cleanliness and water temperature also play vital roles in the balance of the aquarium.

What size filter are you running on this tank? Do the plants in the tank gently sway in the flow? When's the last time the filter was cleaned?

 

[AaronT]

The increased fishload could certainly be throwing off your dosing. I've noticed the same phenomenon when my Endlers livebearers start becoming overpopulated. In the meantime try dosing more phosphate to balance out the nitrogen to phosphorus ratio.