ooooh... it is most likely cyano
bacteria. Does it seem to 'spread out' in the day time? I think everyone's tank goes through at least one bout of that when they are setting up. It is hard to get rid of, what I ended up doing was skimming the tippy-top layer of crushed coral with a net to catch it, and then boiling the sand to kill it off. THat seems like such a desperate measure, but it had gotten so bad despite water changes that I scrubbed my star polyp mat with a toothbrush to get it off. (They are a heck of a lot happier now!)
Hope yours doesn't get so bad.