I would drain it, fill with water. Drive a few days, repeat. Once it looks clear go with cheap walmart universal green and water. I don't like the non green conventional coolants, when green is proven.
Walmart green is Dex-Cool, just dyed greenish yellow and no GM approval for warranty service.
The OP shouldn't have a problem with using "universal" antifreeze, as long as it has 2-EHA and sodium neodecoanate in it. NO G-05.