There isn't much of an odor from the old coolant, it still smells mainly of glycol. Viewed close up you can make out some larger 'floaters' that look very much like bacterial slime colonies. Whatever is growing in there is tolerant to a fair percentage of Glycol.

A tiny amount of the grot might be from nickel oxide (from the plating on the barbs), and there could be some radiator flux in there too. It's not like any number of flushes are going to get it all out - but it's hard to imagine either of those things contributing that much cloud.

Just a theory, but with pure distilled "a few drops" of biocide aren't going to do anything worth mentioning to stop bacterial growth: these bugs are hard as nails.