Either coolant gel from contamination, silicate breakdown/drop out, some iron corrosion or aluminum corrosion. Silicate dropout (white gel, goo or chunks) happens when the antifreeze/coolant has been used beyond it life expectancy and the additives break down. Mixing different types of coolants can make some weird gel byproducts too. Or possibly some form of stop leak. Flush it well and use good quality coolant with soft water or filtered water.
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1968 Camaro SS L78 LeMans Blue original drivetrain
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