BGA, the scourge of the planted tank. It can be caused by any number of things or any combination there of. Low Nitrates and poor water circulation seem to be some of the major triggers.
If it is still a relatively small amount I wouldn't treat the tank with antibiotics, as they will have an adverse effect on your bio-filter as well.
For a small outbreak of BGA IMHO it's best to treat it with hydrogen peroxide. Put some in a syringe, if you can't get your hands on one the small ML doser that comes in the bottles of flourish iron will work also. Directly applying the peroxide to the BGA will kill it.
Be careful not to do to much before a water change. Often this treatment needs to be done over the course of a couple of days. If over dosed, the peroxide can damage gills as well as seriously dropping your PH. As long as you don't go over 1ml per gallon you will be safe.
What ever manner you use t kill it make sure to manually remove the dead BGA ASAP as it becomes toxic after it dies.
Keeping nitrates above 0ppm and increased flow will help prevent its return in the future.