Weird grease in coolant overflow tank

[JakeinAZ]

I have an aftermarket opaque cooland overflow tank, so its easy to see what is inside. There is a slight grease line around the tank at the coolant level there are also pinhead size floating specks. The substance is greasy and a dark grey color. I was wondering what this is and what may have caused it. There are no blown gasket symptoms, no white exhaist no foam in oil and no coolant loss. Any ideas would be helpful. Thank you.

[eliaschristeas]

was this a new or used tank? the used ones almost always are from off road casualties and have calcified mud and coolent in there. if you didn't wash it before instaling it, that may be your problem.