I am with WyldFya. I had this same problem about 6 months ago. The whole tank was covered in BBA. Start with increased CO2. My problem turned out to be too much phosphate and a lack of nitrate. Once I got everything right, the BBA died off extremely quickly.
Spot doses of Excel worked for me. The BBA died and turned white. The problem is that the dead BBA stuck around longer than the BBA I left alone. YMMV.
Brandon