Looking online, some suggest it's from a leaking battery. Since this happened with both the old and new battery, I don't think this is the cause.
Some posts suggest it's from overcharging. I installed a new NAPA alternator around 9/1/2007. My radar detector always shows ~14V while driving around town. That's normal voltage, so don't think that's the problem either.
Some posts suggest it's from a constant draw while the car is off. I do/did have an alarm system (Python 700).
I do use the felt washers on each terminal, which some posts suggest it helps to prevent the white stuff. Haven't used any sort of dielectric grease though.
I scrapped off some of the white powder before taking the picture, but you get the idea of the growth:

Is there any way to prevent this from occuring?