Like Randi and others above, I only see the hard water/calcium buildup. The more I scrape it off, I think I do tend to scratch the surface and so it looks worse! Could it be you are seeing mineral deposits rather than mold or mildew? It gets rinsed out so often and the water flows, so it does not seem likely to me that mildew could form?

Every so often, I take it all apart and pour bleach on it, then scrub with a toothbrush.

The fountain I have now is a Drinkwell and it has a grey textured surface that tends to look better than the shiny white Petmate fountain. Still gets that gritty water line though! (our water is VERY hard even with a softener!)