I'd recommend a citric acid flush.

After finding the green stuff in my 300D, I drained the cooling system, flushed it several times with water and then with a Zerex cleaning product (which I left in for a week) and finally again with several more rounds of water. Just as things were looking pretty good, I decided to do a citric flush as well, just in case.

Well, when I drained that, the stuff that came out was all the sudden bright green again -- as green as when I drained the system at first. I had to flush it several more times with water before it stopped coming out green. It seems like that green crap must have been clinging to every surface of the system and required the citric acid to finally scrub it out.

Also, when draining the system, make sure to use the block drain as well as the radiator drain; otherwise the majority of the fluid remains in the block.