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Coolants don't have detergents, it is likely something like silicone or something. Additives dropping out of suspension whatever it is. Run 16 ounces of RMI25, it will remove them and deposit it in the overflow tank, the tank traps that stuff. Then drain off the over flow tank. If you have the black tank you will need a flashlight to see the slime, and if you don't want to remove the tank just use a wet dry vac.
How many miles on the coolant, I assume it's HOAT? Ever changed it? Ever add water?
You still have power train warranty probably, have them look at it. I don't know that is not the rule or the exception. I wonder what other guys who have replaced t stats looked like.
Was it rough at all? slimey?
That is the paint/corrosion resistant coating peeling off of the coolant neck. New, they are a nice gloss black color. After a while, when you remove them, the inside is bare metal. You caught yours in between. Yes there are flakes of that swimming around in your coolant, no it's not going to damage anything.