Thanks. Searching for the name reveals the images which is identical to what I see in my tank. It seems to be growing pretty fast. What could be the cause of it and is it bad? How do I eliminate the problem, if at all this is a problem?
The cause is some tiny bit of introduced plant. As to whether it is bad or not is subjective; some people grow it on purpose. While it will consume nitrate, it grows quickly and is considered invasive. Any "root" left on the rock after removal will grow back.
As a means of propagation under less than ideal circumstances, it can turn all of its cells into gametes and flood the tank with organic matter (reducing water quality). The greater amount of the macro the more dangerous the risk. Most people who aren't growing this species on purpose remove it because it can be difficult to keep in check and is prone to sporulation.
As for removal, I think most people suggest aggressive, careful, manual removal. Some large herbivorous fish also eat it.