is there anyway you can just cut off the infected leaf? also if you have excell if you can just dose it right on top of the bba that can really help. I have read this can be an effective treatment
http://www.plantedtank.net/forums/algae/20172-excel-treatment-bba-experiences.html
http://www.plantedtank.net/forums/algae/20172-excel-treatment-bba-experiences.html