Using a razor blade is your best bet for calcium deposits like that. Glass is VERY difficult to scratch and impossible to stain so even though it's hard to remove...it WILL come off because it's sitting on the serface. Keep the blade level with the glass an at a 45 degree angle to the glass....Follow up with white vinigar and water in a 50/50 mix.
Soak a rag or paper towel in vinegar and leave it on the mineral deposit for several minutes. Scrub and repeat until it softens up and comes off. "Lime-away" or another brand of hard water remover is a bit more potent (and toxic...), but vinegar will work with patient use.