If you have Excel, then that works well. If you don't have a syringe available, pour some in the bottom of a tupperwear container and rub the top of the leaves in it. The algae should be gone in a few days.
if you can remove the plant without too much trouble just take it out and dip it in water mixed with a bit of hydrogen peroxide. Wash it off with water after and place it back in the tank. Next morning the BBA should have turned red, it will fall off shortly after
trim off the most badly infested leaves then do a 3 day blackout, with large wc after before you begin, and after each day. overdose excel (use their 5ml/10gal dosing after each day, unless you have plants sensitive to excel--anacharis, vals). lots of work, but worth it in the long run.