Got as much "milk" out as I could via siphoning and through the fuel pressure gauge relief valve.
What's left, at worst, is a cloudy yellow. The "milk" isn't flammable. The cloudy yellow is.
I put about 5 gallons of fresh gas on top of the yellow cloud. Hopefully I can burn through that in the next few days.
No idea what the "milk" was, though. It wasn't water. Water isn't white and the milk wasn't as viscous as a liquid. It was somewhere between liquid and gelatinous. About the viscosity of thick oil.
Maybe it's just been gathering in the tank over the last few years. And then again maybe I need to get a locking gas cap.
I'll do a little looking around to see if I can find out what it is and post back if I find anything.
Thanks again.