Blue-green algae generally develops in low, or non-oxygenated areas within the tank (with no, or poor, water circulation). There are other causes as well, but once you get rid of it, improving circulation within the tank is a main factor in keeping it from recurring.
Here's what has worked for me, quickly & effectively:
Turn off your filter, and siphon out water to bring the level down so that you can easily get to the bacteria to treat it.
Dose all of the affected areas with (H202) straight hydrogen peroxide, at close range, using a plastic syringe or test kit pipette. You should soon see the affected areas get covered with air bubbles (oxygen). Refill the tank, leave the filter off for up to an hour to allow the H202 to remain in place and do it's job.
You should then see the air bubbles floating up into the water column, a sign that the H202 is working.
The following day, most or all of the BG algae should have been destroyed, but if there are still traces, repeat the same procedure once again.
Following this treatment, you need to improve the water circulation within your tank, using a spray bar, power head, or a small circulation pump, or some other similar piece of equipment, along with stepping up your tank/substrate cleansing routine, and water changes.
Sure worked for me, in 2 separate tanks over the past few years.