Wait for others to chime in since I'm not 100% sure this is BBA.
If it is BBA then I would trim off heavily affected leaves and then do a h2o2 spot treatment. Do a small test first on one or 2 leaves and see if it works. Use a syringe or turkey baster to spray h2o2 directly on the algae. Only use 3% hydrogen peroxide! It doesn't take much so spray lightly. 5-10ml is all you should need for even a large spot treatment. Don't use more then 1ml per gallon. TURN OFF YOUR FILTER before adding any h2o2 to the tank. Wait 15 mins or so and then do at least a 50% water change and then turn your filter back on. Over the next day or 2 the algae should turn pink or white and die off.