If that is all that has grown in 2 years I wouldn't worry about it too much. It does look like a possible algae; I would just keep an eye on it and see how fast it grows/replicates, if it gets scary scrub the rock. Meanwhile, its interesting.
he did say it was burried in the back of the tank. it could be that it wasnt getting light. but if that rock has been there for 2 years, i doubt that that is the only one in there. there must be more. either that or spores were released with the last fish/coral/invery you bought.
i looked around in the tank when i first moved the rock up front. i did not find any growth on other rocks. i have a very small amount of coral in the tank. some xenia that are sprouting,green striped mushrooms and a haitian anemone cleanup crew etc... all water paramaters are good.