[PapaBear]

Your mineral content and ph are very comfortably on the low side. Can you possibly get a good closeup picture of the white deposits?
I'm thinking as I type, so this may be a bit of a ramble.
If you had a phosphate problem you would probably be fighting a running battle with algae, but you did not mention green water or string algae.
Iron would leave a reddish brown stain like rust, so that is unlikely as well.
I don't know if high zinc or magnesium levels would cause this or not, but I think your gh would be higher if that were the case. I could be wrong about that though.
Lime content high enough to leave scale deposits would probably have your ph running much higher than 7.5
One possibility that comes to mind is a strain of freshwater algae I hadn't heard of until recently. It does leave a calcified crust on surfaces when it dies, but I have no idea if that could be it.
You mention that your water supply is from surface water. Is it a private source pond, or a treated city water supply from a lake?
Maybe some of our resident water chemistry pro's could chime in here.