Check your phospate levels!!!!
Ben, have you tested your phosphate level in your tank and tap water? In West DM here one day the phos levels exceeded 10ppm. Artesian wells around here are high in Phos. My tank routinely test over 5 (that's all my Hagen kit goes to), it's never anything less than dark blue. I think you are supposed to let tap water sit for 2 days before testing to get an accurate result, which sucks because levels can change daily.
I had huge diatom brown blooms that started to turn into green hair and maybe even blue-green. I was told to either use RO/DI or Phos Zorb or products like it. No easy way to fix it unfortunately. I have swords that are completely covered. My back glass was so bad I had to set up my Magnum 330 with the diatom filter cartridge, charged with diatomacious earth, and scrape it, then vaccuum the gravel and let the mag run until it pretty much clogged up, which only took a few hours. It was na-a-a-a-sty.
Even my SAE, 3 mollies, 3 swords, 5 platies, and rainbow shark couldn't keep it at bay. They took prety good care of the brown diatom, but didn't seem to keep up with the green stuff, and it looked pretty gross so off it came (except for about 1 sq ft I couldn't get to - tank too close to wall OOPS).