Some algae looks like that. That would be my suspicion. Try adding some antifreeze, hyperlube, and non-chlorine/non-bromine hot tub sanitizer to your coolant after doing a full system flush (to kill it all). Check out how to do this easily here:
http://www.procooling.com/articles/h...atercoole5.php
No scrubbing involved and it will ALL die. Plus, any non-organic gunk will get flushed as well.