scrubbed the heck out of it, and reinstalled.
It's now back, just as bad. I measured the voltage. It's 12.6V off, 14.2V running. That's all indicative of a properly functioning alternator.
The battery is 5 years old, and an Optima Yellow Top. It's a sealed battery, so I think gas is leaking from the negative terminal, causing the corrosion. That would not be good.
So, my analysis is the alternator is fine, the battery is likely leaking. It's time for a new battery.
Is my conclusion right? Is there anything else I should be considering? I'm not excited to drop $300 on a battery.