i assume its bba.. there for.. Unstable CO2 levels will induce BBA!
The best way to combat Brush algae is by maintaining CO2 at 30ppm, nitrates at 15ppm and phosphates at 0.5ppm. Leaves that are badly overtaken should be discarded. Observations; i have same problem, i have been told to dose 1ml per 50 liters of Easy Carbo / Flourish Excel every other day for a week. The algae will turn purplish/pinkish then disappear. Maintaining sufficient CO2 level and is the best way in controlling this algae.
Siamese Algae Eater will eat BBA. ( but there few fish looks same.. only true SAE eats BBA)
Also if the BBA is located on small sites, you can use Flourish Excel, or Easylife Easycarbo with a syringe directly to that site (with the filters turned off whilst you do this). It should dull down and turn grey, and then go from that particular site.