Thanks for your thoughts, everyone. After your comments, I'm thinking the culprit is too much light. The tank does get ambient light from a north-facing window 10 feet away, but the biggest light comes from the tank itself - I tend to leave it on for a good 12 hours if not a bit more. I turn it on when I get up and turn it off when I go to bed at night.
Yeaaaah....so that's probably my problem right there. My excuse was that with two fish tanks on one powerbar I didn't have room for the timers to plug in. However, I could get another powerbar and problem would be solved.
EvilVOG - oh, I'm so sorry to hear BGA has invaded your tanks. That's totally sucky :/
It doesn't smell bad, which is good.
I went looking for pics and found hair algae seems to be the closest match:
Though I'm not 100% sure on the ID because it's growing on the walls, too. There is a green film on the top of the water as well. Initial research tells me these aren't hair algae attributes, but it could be the sources I found just don't mention it. Anyone have experience with this type of algae?