Hi Quicksilver.
Regarding the "white powder" on both the zinc plugs, and the end caps of the heat exchanger, it's hard to say what it might be, other than either a product of corrosion between the parts and the CU-NI heat exchanger, or salt precipitation from the raw salt water circulating on the cold side of the heat exchanger. As the raw water absorbs heat from the fresh water on the hot side of the heat exchanger, the salt tends to evaporate out and settle after shutdown almost like a high-tide mark on the beach inside the heat exchanger. In some cases, it can form a bridge clear across the intake. This might be showing up as your "white powder".
I would therefore recommend removing the inspection covers on both ends of the heat exchanger, and determining the health of the exchanger itself. You may find this salt precipitate, products of the zinc erosion, other "stuff" possibly blocking the heat exchanger tubes. If so, the best fix is to remove and have all the heat exchangers on the engine professionally cleaned and pressure tested.
An interim do-it-yourself fix can be accomplished by first manually "rodding-out" the tubes with a small flexible probe. Next, rig a 500 gph bilge pump in the bottom of a bucket, plumbed to pump phosphoric acid into the raw water cooling system just downstream from the raw water cooling pump (remove the zincs first!), through the raw water system, and back to the bucket as it exits the engine enroute to the exhaust system.
Let this thing percolate, boil, and bubble for several hours, and much of the "stuff" that lives in the raw water cooling system will end up in the bucket. Use a new gasket on both ends of the heat exchanger after re-assembly, re-attach your raw water cooling hoses, fire the engine for a few minutes, and you should be good to go, for a while at least.
Sorry to be so wordy. Hope this info helps.
Pete