Brown, sort of powdery algae that craps up everything in the tank. It wipes off easily, but (too late) I read that scrubbing it actually spreads it. It's common in a new tank, but eventually fades away, plus a clean-up crew helps.
The tap water was evidentally helping mine along. Eventually after enough water changes, changing some of my filtration setup, and cutting back a little on my feeding, they went away.