I've been putting off searching for an answer to this for a while now but I pulled the head off my old Z6 block a while ago and found these "boogers". It's actually a sort of gel-like substance. What I'm wondering is what exactly causes this?

A couple notes about my old motor:
-It was leaking coolant from around my waterpump. I kept adding water and coolant of two different brands. Both were rather diluted since I often just added water to keep the coolant level up. Some info here.
-My motor saw rising temps a few times when my coolant ran low.

All sorts of ideas are welcome.