I hear ya on this, everyone keeps saying its because of flow, but if I added any more, I would have a tsunami setup. Flow helps, but it is from something else.
Like it was said before, it feeds off of dissolved organics, but I have cut back on feeding A LOT, and it was still there. This weekend I took all of the rock out and 'washed it' in clean saltwater and did a huge water change and it is still coming back.
Old bulbs are sometimes quoted as the problem, but mine are only 5 months old (T5s), so again......
I truly think that it just occurs in tanks sometimes, as cyanobacteria naturally occurs everywhere in nature, and we just need to find the quick fix to get rid of it. I haven't used a chemical yet b/c they are said to kill more than you intend, but I'm about there.