I was looking around my engine bay and I noticed the radiator coolant was above the "HIGH" mark in the reservoir. So i took a pipet to decant the fluid so that it is below the "HIGH" mark. As I was pipetting I noticed that the coolant was green and very murky with white particles At first they look like sand of some sort, however I am uncertain.

I took a picture of it, can anyone advice on what the bottom layer (white) is and should be alarmed? This is a picture of the coolant after sitting overnight and the "stuff" have settled to the bottom.

 

are you the original owner? Or have you ever put stop leak in it? I know sometimes those stop-leak products will leave nasty residue.

 

I am not the original owner. I have never used stop leak.

 

Although someone else may have a better answer I'm gonna assume the previous owner had used a stop leak. Either that or they dumped a bag of sand in it for the hell of it. Either way that looks pretty nasty, especially if that was only a small fraction of what was in it. If I were you I might consider a good flush just to be safe.