What is this gunk in my coolant passages? (pics inside)

[zads]

I don't remember this crap being there when I changed the water pump,
and I've only run it for a very short period of time since then.
I don't want it to clog my brand new Koyo...

I've never put radiator leak-stop stuff in the car before,
but the previous owner may have.
The stuff is pretty gummy, almost snot-like.



Any ideas on what it is,
and how to get rid of it besides flushing it out with a hose??