What is this?

[HotRoddin]

Originally posted by Kalyn
Whatever it is, it does not mix with gas and appears to be some kind of liquid.

Its most likely water ... it lays in the bottom of the tank (heavier than gas) and picks up the corrosion on the bottom of the tank. It doesn't have to have flakes.
You probably have a bunch of it in there ... best to have it professionally flushed ... if its really corroded they can get rid of the corrosion and seal it.