Look like brown algae on old growth. Get more algae eaters or dose Excel.
Excel is used as an Algaecide that has nothing to do with pH drop. The liquid carbon claim has no proof.I don't think I would add Excel when you already have over a 1 point PH drop from real CO2. You should take a look at your dosing and nutrient levels. Flourish is not comprehensive plant nutrition. It's basically micros and trace elements. It does not have sufficient levels of the macro elements and depends on other methods to supply them. If that's all your feeding the plants you need to figure out where you nitrogen (N), phosphorus (P) and potassium (K) levels are. One or more of these may not be at a sufficient level for the amount of light you're adding.