Goo in radiator

[acsho]

I have been noticing a rubbery goo in my radiator, it's just around the neck where the cap goes and it is a blackish brown color. This weekend I changed the front motor mount, so I had to removed the bottom hose and refilled with fresh coolant; I also drained the overflow tank and I noticed the goo is in the tank too but just around the top inch of the tank.
Does anybody have any idea what could be causing this?

 

[Kens1992mtxSHO]

Sounds to me like it's in need of a flush. It happens with old coolant.