For what its worth I have managed to melt Alternanthera Reineckii Mini in the past when spot dosing adjacent anubias with Excel to get rid of BBA - it seems to be particularly sensitive to glut. As a plant, it was also was also more than happy to suddenly shed it's lower leaves if they started to get shaded - your images looks as if they older leaves are most affected by algae. Also as others have said, Amano's are known to consider them a delicacy if you have them in the tank.
If it were me, I'd rip the Alternanthera Reineckii and replace with something a bit more robust. If you want to treat any persistent BBA on other plants like the epiphytes, spot dose it with glute with the filter off for 15 minutes using a syringe or pipette, but don't exceed the full tank dose. I always found tank dosing much less effective at eliminating BBA than spot dosing.
Finally, stop wasting time measuring water column nutrient levels, and just dose EI levels of ferts, given how heavily planted your tank is. The ferts don't cause algae, so you can add them without concern.
As
@John q mentions, you are running quite a lot of light. You don't mention what percentage you are running the two C2 lights at, but despite their small size, they are quite bright lights at 20W / 1580 lumens each. if you are running them at 100%, try dialling them down to reduce the CO2 demand. I have one running on my shrimp tank, granted its low tech and only 20 litres, but I've found 15% with plenty of surface plant cover more than sufficient for the epiphytes I have in there.