Remove the worst leaves and try spot treating with Excel. Hopefully when things stabilize it will stop spreading. The number one cause is bouncing CO2 levels.
if its bba, if it gets on wood/rocks/substrate, you can kill it really fast by rinsing it under HOT tap water. you can also kill it more gently and slowly with hydrogen peroxide. other than that, do dose with excel and remove as much as you can with your hands