Sounds like dino's then. I've had the stuff crop up in my 50 gal tank twice now. Only way I've ever been able to get rid of it was to siphon out as much as possible then black out the tank for at least 3 days. And I mean completely black it out, cover it with black plastic or something, don't let any light in. Be prepared to empty your skimmer a lot during this time. All that dying algae is going to have it working over time.
After the blackout you'll be amazed at how clean the tank is. but do an immediate large water change. All that dead algae will cause a nitrate and phosephate spike and you'll be right back where you started otherwise.
and bring the lights back up slowly, 2 hours the first couple days, then 3 for a couple days and so on till you're back up to your full photo period. That way you don't photo shock your corals.
Good luck, this stuff is a real pain.
Phil