Been there, done that.
For me it was a phosphate problem - elevated levels due to incorrect additives I believe? (It's been a few years now...)
Had green water for about 6 weeks in my 90. Ran the Kent phosphate sponge in the sump. Borrrowed a UV sterilizer and some Maxima clams (they eat and grow strong on the stuff), no water changes - agree with Rikko here - and minimal lighting. Tuned the skimmer, ran carbon and floss (changed it often).
It took a while, but when all the measures were finally in place, it cleared up that week.
Tried the Al-B-Gone or whatever that stuff is called, too - didn't seem to help. But the clams grew!