Chemicals like lime scale and chlorine are the main culprits for the cause of dots on your glass, otherwise more accurately described as lime scale stains. When water droplets evaporate the chemicals remain on the glass and this is what ends up staining the glass. To avoid this have a squeegee in the shower and give it a quick clean after showering or at least once a week to prevent permanent stains or 'pitted glass'. Thicker stronger better quality glass does not stain as easily, keep this in mind if replacing glass screens . There are various products that we use to remove stains but only to a certain extent depending on the age of the damages already caused !