No ****, really?! I've been going months with the impression that antifreeze is a good enough biocide! I mean if it's toxic enough to kill me if I drink it, shouldn't it also kill water critters too? LOL

I learn something new every day. Thanks for clearing that up.
Yeah those growths appeared only in my system and not in the jug of backup coolant I have sitting on the floor. Could be related to sunlight (my system is an external box with hoses running into the case), but I suspect the radiator was dirty. I bought it from a surplus store and those places aren't exactly the cleanest.
- Jerky.