Might this be what you are speaking of?
Everyone is right. Flush it well. This is dexcool that has been left in too long and has either come into contact with good old green stuff or air. Once this happens it's actually a corrosive substance that eats all the good little plastic parts (namely intakes) that GM uses. It wasn't as maintenance free as GM advertised.
Best bet is to flush it and use green. You can use dexcool again with no problems as long as you keep an eye on it, but I prefer green anyway.