Residue in coolant reservoir—what is this?

I was about to fill up with coolant after a w/p replacement and notices some discoloration, so I pulled the reservoir out to get a closer look. What’s going on here, guys?

http://audirevolution.net/addons/albums/images/899593

http://audirevolution.net/addons/albums/images/251745

Car has ~106k miles. The coolant reservoir was replaced at ~65k miles. No oil loss was noticeable. Not noticing any blue smoke at the tailpipe. Please tell me this is something other than a head gasket…

Suggestions for cleaning this out so I can get a good baseline going forward?