These stains re most likely from hard water. Stains from hard water can occur when water evaporates and leaves behind trace minerals such as calcium and magnesium. If your shower tiles are not sloped correctly and not draining freely, then water could be remaining on top of the tiles,
Hard water stains can be quite stubborn, so you might need to repeat this process several times.
Remember to wear personal protective equipment, including safety glasses, a face mask and gloves, when cleaning. It is also wise to ventilate the room well while working, opening windows and doors if possible and using an extraction fan. - MitchellMc