[zoson]

Stop using antifreeze. I would think that it's viscosity would be terrible for a block like the WW. Secondly, you need to give us more detail about your system. Pump, metals in your loop. The discoloration could be algae, or it could be oxidation. Both of which would harm your performance. You really should take the time and flush your entire system to ensure that the gunk is not building up in your radiator (which could be potentially disasterous for your computer.)
Depending on what the substance is, methods of removal will differ. If it's algae, just run some hydrogen peroxide through your system for several hours. Be sure to use a HUGE reservoir like a bucket or something so that there's an area for the gunk to collect outside of your parts.
If the gunk is oxidation you're going to need to sand the block to remove it, or get some sort of etching agent. Most people in here would advise against using an etching agent to clean oxidation due to the difficulty controlling the etching agent.
-Zoson