We need better pictures than that to judge what it could be. Without being able to see it clearly, anything is speculation. Can you get your hands on a DSLR? Preferably on a tripod and maybe even with a macro lens.
How long has the loop been running like this? How long have the QDs been in use and have they seen any other coolant? Do you still remember what ratio you used to mix the coolant? Unfortunately there's no information on the Mayhem's website about what exactly is in the coolant - but from what I have read on the topic, I believe that this could possibly be corrosion due to an insufficient amount of corrosion inhibitors in your coolant mix.
http://www.mayhems.co.uk/front/xt-1.html#.UsG0JvTuLRy
Edit: It could also be mechanical damage to the plating from turning the QDs when closed. Without close-up pictures and the knowledge of what to look for, it's really hard to tell. If you really want to know the cause of this, maybe dig up the old thread about plating problems with EK blocks and PM some of the more knowledgable people who posted there (when you have better pictures). The report EK commissioned on the nickel plating issues was also quite insightful:
http://www.ekwb.com/shop/EK-IMAGES/E...nal_Report.pdf
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