Sorry, my tank is just a regular All Glass 20 high. So I can't really answer about the scratching part.
As far as the mottling and encrusting, it's just like any surface that sits without being cleaned. If you leave it long enough alone, pretty soon you'll have purple, and green coraline algae all over it. I personally don't like coraline algae to a great extent, I even scrape the back of the tank occasionally to get the stuff off. But I don't mind it so much on the faux sand bed, and I'm happier with the faux bed than I would be with just a glass panel bottom or acrylic bottom.
Aaron