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The JP3 site seems to indicate those bearings are stainless steel, or have subtantial stainless parts in them. If your rudder stock is aluminum, then the white deposits you see may be the result of dissimilar metal corrosion. Putting aluminum and stainless together in an electroylte (salt water) is generally considered an extremely bad idea. Even with gobs of TefGel or other corrosion-preventive goop.
IF DuFour spec'd it that way...I'd assume "It seemed like a good idea at the time" to someone there. But you might want to check with the factory to find out if that really was their idea, and if they use anything special to prevent problems with it.
And needless to say--if it is not calcium, the cleaners you are using probably won't do anything, you'll have to take a whole other solution to it. (Pun intended.)