I just gently squirted flourish excel on the effected algae area when overdosing with exel and it turns it red almost instantly and then turns white and is gone within a couple of days
This is called "spot treating" with excel and its a widely known technique, turn your filter off when you do it for best results, switch it back on after about 10 minutes or so to let the excel have a chance to really penetrate the affected area of algae, this method is very effective with bba algae, works every time.