Bluegreen algae is a cynobacteria ... which thrives at low light so in this instance lowering light won't help. It will outcompete green algae at low light levels. Ironcially raising the light will help. Eurythromacyn (sp.?) is pretty effective but overkill and a last resort. Psyco's suggestions above I'd deffinately try first before resorting to meds.
This algae sure grows fast. I will just siphon out as much as I can with each water change. I know the sand is really dirty. This tank was over-stocked and under-filtered for a long time. I might just siphon the sand out too.
I hope I can keep it from getting started in my other tanks.
A good way is to "quarantine" this tank equipment wise. If you have something pesky in one of your tanks and you're using the equipment on all of them, it's only a matter of time before it gets a hold on the others.