Thats Algea or dye or something seperating in the liquid. It's not oxidation. Oxidation is black. Oxidation doesn't grow on tubing. Scroll down to see the oxidation on a CPU block in this build log, about 1/2 way. It is also a log you can use on how to rebuild your loop. You need a 100% teardown of everything, everything.
http://www.overclockers.com/annual-w...build-journal/


You don't mention the use of a biocide at all. You were using something to prevent growth I hope?
And you said two years? Did you keep the same liquid for two years?