Hi Swampman.
It's Black Beard Algae (BBA).
I have had it in my tank before. Remove the leaves most badly affected if you can. Then treat the BBA directly with Glut or Excel (some people use peroxide but I haven't ). Work out the dosing for the whole tank, so that you don't over dose. Turn off your filter and any other device that produces water movement. Wait a couple of minutes. Using a pipette or syringe slowly squirt all the Glut or Excel over the algae. Wait 5 minutes and turn on the filters etc. The areas successfully treated will turn red by the next day and then slowly melt away. It's not normally possible to do this in one go and may take many days. Be persistent and you will eradicate it.
Next find the cause. Not so easy as its one of the trickiest algae a to tackle. Water management is essential, keeping a low nitrate level helps. Co2 injection helps but if you don't want to go down that route then a daily dose of Excel or Glut is a good deterrent and the plants will appreciate the carbon source. It's very much a case of balancing the water quality, light, Ph (lower Ph encourages this algae) and fertiliser regime.
I found that the more plants that I have the less algae I have.
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