It's likely that a full clean is needed to ensure nothing is growing or building up inside the blocks where you can't see. That said, what coolant is it?
I had some Alphacool Cape Kelvin Catcher fluid that turned slightly blue within days, and slowly went more blue/grey over 3 years until it was a murky colour (but still transparent).
I left it running that long as apparently it was the anti-corrosives reacting to pH conditions. Temps never changed, blocks were almost as clean as new after 3 years.