Brombal

Thats not corrosion.
 
Copper tarnishes first (gets dull and darker), then can eventual go blue/green.
 
Thats likely residue from manufacturing, likely form the radiators.
 
Just take apart the block give it a quick clean then put the loop back together, do the same for ur CPU block as that likely has some buildup as well.
 
Once done and the loop is back together make sure u also add a biocide and corrosion inhibitor to the distilled water. Distilled on its own will work but u will eventually get corrosion, i know i ran a loop for 4 years with just distilled and my blocks corroded (not a problem as they were Opaque blocks). If the loop is in direct daylight it can also start to grow algae if u dont have a biocide in it.
 
White vinegar is the usual method of cleaning out rads, brown also works its just not as easy to tell how much crap u've got out the rad. Fill it, leave it for 30min - 1hour , then flush and rinse thoroughly.

I'd open it up first and check what it is. Doesn't look like corrosion to me. Looks like oily residue from manufacturing. Did you clean the loop before assembling it in the first place?
Also mix some biozide into the distilled water when you refill it, could be algae that is growing. You can clean it with a tooth brush and some whitening paste. If you can get your hands on it a metal polish like Autosol works even better.

You can flush the radiator with distilled water and nothing added to it. However I prefer a very thin mix of distilled water with some G12+ Antifreeze (1:10). Then refill with some distilled water and let it cycle a until the antifreeze is out of the loop. I have no experience with citric acid but I'd be careful because it does react with the copper.