whats this fuzz growing in my new radiator?

[rotorwrench]

No mater how pure the water is, it has no anti corrosion or anti fungal properties at all. Aluminum is a dissimilar metal to the radiator and the block. With no protection in the coolant, fungus can grow. Spores can come from the air so it doesn't matter how clean the parts or coolant is.

I'd recommend at least running a water soluble oil in there but just plain old anti-freeze would protect it about as well as anything.