Poor cooling water treatment. The pipe work is rusting to form Black Iron Oxide (due to lack of Oxygen) which is dissolving in the circulating water and being deposited by Galvanic action onto the Heat Exchanger Plates which are less re active than the steel pipes.#
Same as the steel Hull corrodes, and tries to plate itself onto the Bronze Propeller, but is constantly eroded by the turbulent water flow, over the prop.
You need to power flush the system out with detergents to clean it and then maintain correct cooling water treatment.
It is always better to ask a stupid question than to do a stupid thing.