Definitely algae. Maybe blue-green algae (cyanobacteria), which would explain its growth without light. It selectively grows on the sides, there's an interface boundary at the air mark, there's dead sediment at the bottom of the reservoir. Owning a 100 gallon aquarium, I'd recognize the filaments any day.

Y'know, looking online, it's actually pretty common, it seems:
http://www.google.com/search?q=algae+brake+fluid

Just weird.