I'm out of the loop for watercooling...

I don't know where it is from. I can only speculate, maybe I didn't flush enough? I don't think it should be from the premix, unless it is reacting with something left in the loop. The annoying thing is I'm not sure how best to clean it. Take the blocks apart? Or might a bit of reverse flow dislodge it?

Drain, flush with distilled and refill premix is my only guess at this point unless you didn't clean out the gear prior to installation. Also, check the date/exp. on the premix if its too old.