I would suspect that white stuff to be .. err .. Kalk .. sorry, just realizing I don't know the English word lol, calcium carbonate by chance?

Have the same on my Oscar tank atm because the Oscar splashed a lot of water out at feeding time, and I didn't move my lazy butt to wipe it away instantly.
Else occasionally using window cleaner as Penn said, and for really resistant cases I've used something with a razor blade, just gotta be careful not to get near the silicone. Just thinking I could possibly try such a thing I use for cleaning the ceramic stove top sometime.