Is there such a thing as grass algae?

[asylumdown]

In the 6 months since I set my 90 gallon reef up, one of the rocks, which started out looking like a cluster of tube skeletons, went from having brown slimy bits on it (how it came from the store) to being completely covered in what looks like the lushest, greenest lawn ever grown. We're talking millions of tiny strands of dark forest green algae that grows to a near uniform length of about 2 cm and is so soft and resilient that when I tried to push a frag plug into what had previously been a deep crevice in the rock structure it actually bounced out. it doesn't look like any hair algae I've ever seen, more like an actual plant or finely manicured golf course lawn. At first I was delighted because it was so pretty, but now little patches of it are starting to take root elsewhere in the tank, mostly in places that are slightly shadowed or don't get the full brunt of the metal halides. I don't know if it's something I should be concerned about, my tang nibbles at it on occasion but it's definitely not his preferred food. I run a phosphate scrubber and have never been able to get a color change on a nitrate or phosphate test, so I don't think it's a nutrient problem. Should I be worried? It really is quite pretty...