Ominous bitty mud like substance in coolant

[salvy]

Good afternoon ladies and gents,

I have an ongoing 986 project that has been of the road for two years now.
Around six months ago I used a coolant flush on her and flushed that out with tap water and flushed that with de-ionised water.
I turn her over and leave her to run up to operating temperature regularly only to find the coolant reservoir contaminated with the substance in the pics.
It doesn't feel greasy as I'd expect from oil and looks a little bitty.

Does anyone have any idea?

Much appreciated.