That almost certainly looks like BBA to me. BBA has two major forms: the little 'tufts', and a much flatter form that looks like what you have, usually starting at the edges of a leaf. I see it present like this in my tanks sometimes.
Your problem is almost definitely CO2. I say this confidently because I see at least two other majors signs of insufficient CO2 in this picture. In addition to the BBA and the sheer amount of light you have, you appear to have some form of GDA on your plants, which you generally see when you have low CO2. Further, the way your plants curl in on themselves is indicative of carbon stunting. The leaves of many stem plants will do that when they are put in high light but low CO2. Its a sign of a less than healthy plant.
Also, 0 ppm nitrate is actually very bad for a planted tank, especially one with as much light as you have. That's a time bomb waiting to go off.
CAE won't do much for algae once they start to grow, and they'll become liabilities against your other fish.
