Hair algae at a guess...
From the normal Algae page:
Hair algae is usually found on the plant leaves and decor. It comes in long strands at least 3" long, and in a variety of colours. Again this is an easily treatable algae which will disappear when correct levels are maintained in the tank.
Causes -
A) If its green - High iron. This can be because of - Water source has high iron content, out of balance fertilising, nutrient tab in the substrate leaching into the water column (poking through)
B) If its black - Low co2 levels
Solutions -
1) Increase co2 levels
2) Check fertilisers for high iron content
3) Adjust fertilisation
4) Dose flourish excel
Either by adjusting the CO2 levels or reducing the iron in the tank will get rid of this algae pretty quickly.