Jelly-like substances are often hydrates which are salts with a lot of "water of hydration" attached to the molecules. There is probably a lot of water in addition to the water of hydration.
It may also be a product of an iron based bacteria that is feeding off the iron in the pipe. The galvanizing is probably gone near the water/air interface.
I suggest taking your sample to the water lab that you usually use. They can probably dilute it and then test it like a water sample to tell you what's in it. They probably have some ideas from their experience.