BBA is evil! Can be hard to get rid of once its there. It's just finding the balance between light, CO2, and fertilizers. This goes for all algae.
CO2 could be the issue here. I'd adjust a little. Macro shortage; Nitrates shouldn't be zero.
Staghorn causes are low CO2, Macro shortage.
Are you dosing any fertilizers?
I'd trim the Dwaf Hairgrass down to get rid of most of the BBA.