Looks like one of those Alphacool reservoirs that have end pieces cast from lead.
I'ld bet any money the black nickle plating is lifting away and the lead (or whatever it is) is reacting with other metal and or coolant in your loop...
plus giving off nice flakes of the plating to deposit elsewhere in your loop.
had one of them few years ago so my is from personal bad experience with one
Edit: resized the pic
Edited by fast_fate - 5/18/15 at 2:44am