I am not convinced that it is BBA. The color suggest diatoms to me but photo's can
deceive on color so not saying that I know it's not BBA.
A closer picture of just one of the plants might help/w this. But if it is or not, I'd suggest you split your photo period.
You light is not really as high as Amazon Swords like and I'll bet they grow rather
slowly. But then that could be an advantage to you so...
I would suggest that you look at the factory directions on dosing the Excel and cut it in half for the first week. Vals are sensitive to it, but if gradually acclimated to it they usually adapt. I'd also ignore the directions about using more after a water change and at the start up of using it. Just do half of the recommended daily doses for the first week and then increase to full doses. I use slightly more than 2x the recommended amount(don't have vals) and have no ill effect on fish.
I work nights but my photo period is split as if I worked days because I get home just after people have already left for work. So it fits fairly well anyway.
I'd do 3 hrs or just slightly more somewhere in the A.M. then a dark/no light period of
not less than 4 hrs and then a 4 hr light period again. Total of 7-7.5 hrs.
Algae doesn't "store" energy but plant do so it gives the plants an advantage when
you split the photo period like that.
I don't know the exact distance between your fixture and the sub. But this chart tells me
that you have between 38-43 PAR as a guestimate.
FugeRay 48" chart.
But that is not very high. Just getting into the med range. I have 45-50 PAR/w those same hrs I listed as the split period and my Anubias don't have that/any algae on them.
I use one dose of EI ferts per week and the 2x doses of Excel each day.