TAMD 41B Carbon in Coolant

Hello, I have a TAMD 41B that is running perfectly with no issues but when I checked my fluids I found what appears to be carbon in the coolant reservoir. The fluid level was fine and the antifreeze was green and clear but the sludge type material was on the bottom of the cap. I wiped it off and ran it another 4 hours and found a little bit more. I should say that last year an oil cooler failure caused #6 cylinder to need a sleeve replacement which I did and it has run for approx 70 hours since without issue. I was hoping that it was just leftover contamination but I am concerned and was hoping someone knows of something that could cause this. Once again runs great cool temps, full power, no water in oil. Thanks