Thanks Zachxlutz! The planning continues. And the thinking. The doing part hasn't started, but I'll get there.
No I never got a positive ID. Not sure that it matters. I found so much conflicting info it made my head spin. Wouldn't it be great if someone made an online calculator? You could just type in a description and symptoms, and it would tell you what species you have, and what to do about it. That would be sweet!
Mine do not appear to be the toxic kind that kill snails. It doesn't appear to grow on the sand or from the sand. It grows throughout the day, peaking at night, with lots of oxygen bubbles. When I look at the tank in the morning they are greatly reduced. The UV does appear to be lessening it, after about two weeks. I'm definitely happy about that! I was beginning to wonder if ANYTHING would work.
I don't have a sump. I have a heinous UV setup right on top of the tank, with a pump sitting elegantly in the seagrass. I'm too embarrassed to post pics of it. As part of my planned re-do, I am seriously considering reintegrating it into my closed loop plumbing, to help prevent another bloom. I hesitate because I hate the idea of it killing the free floating live organisms in my unfiltered, live water. On the other hand, it wouldn't get all of it, and benthic organisms wouldn't be affected.