do a water change and gravel vac as much as possible out, dose nitrate and increase water circulation. now do a thee day blackout. dont peek. dont feed your fauna, they'll be fine. after 3 days remove the cover and do another watar change. voila! problem should be solved. now dont allow your nitrates to drop again.
if none of this works you can always use maracyn. bga isnt and algae, its a bacteria. maracyn is an antibiotic and will kill the bga but you have to follow the directions to a T. i definately recommend the natural way listed above