[NSFL] Teal-colored gunk in CPU block. Can someone tell me what caused it? (custom water cooling)

Hey guys, I'm helping my roommate diagnose his custom loop. The loop is clean as a whistle except for the block. It looks like this NSFL, click at your own risk. We use only distilled water with inhibitor and so far, I would assume it worked fine since the rest of the loop is clean as a whistle. We're pretty anal about it and we clean it often and thoroughly so this comes as a surprise to us.

We obviously just want to figure out what the heck this is so we can nip the problem in the bud. I'm surprised it has not spread to other parts of the loop since it's gunk-y. Thanks!

Update: It appears to be oxidized copper but the GPU is also copper and it's clean. Very suspicious.