Hi Bud,
Thanks for posting some additional pictures of your tank interior. I would suggest that leeman's suggestion regarding an attempt to dissolve the material in distilled white vinegar may provide some clarity, and may well shed light on this issue.
A question for leeman-what do you hypothesize as the source of the carbonate ion, which bonds with water, to potentially form a carbonate residue inside a boat's fresh water tank? Carbonates typically are found in such as terrestrial caves, the result of relatively-acidic water dripping through limestone. I'm open to suggestions!
I've asked many folks for their opinion on "stuff" such as found in Bud's tank, but haven't had confirmation of anything yet. Lots of opinions (like mine!), but little evidence. I've been told such deposits could be aluminum oxide, washed off the surface of the tank as it oxidizes. But oxides of aluminum are REALLY hard (they're used as abrasives), and the deposits I've seen in free-floating form inside water tanks aren't. In addition, aluminum oxide is tenacious stuff-it doesn't want to leave it's aluminum home! Other possibilities for Bud's deposits are possibly some form of a magnesium chloride, as magnesium is the major alloying element in the (typical) 5000-series marine alloys, and the chlorine is floating around in city water inside the tank.
So back to Bud's original question-wha zis? My answer-beat's me, but IMO it's aluminum chloride. Given he's got access to the interior of the tank, a thorough cleaning with good old soap and water may be in order, and perhaps an attempt made to dislodge one of the crusticles seen in the tank seams. And an attempt to crush one of his nodules would be instructive as well.
Regards,
Pete