The side of the gunk that sat on the valve(underside) is crusty. That side is textured like mud. The outside that is exposed is smooth and squishy.
Im about 100% sure it a by-product of the e85 fuel. I tear into motors almost daily at work, and i dont ever see build up like this other than typical carbon deposits. There all regular gas motors. And I dont think rubber or anything in the fuel lines would be able to flow through the injectors.