The white film is the residue of hardness and other minerals in the water. When the water is heated or evaporates, the minerals leave a white coating on items such as showerheads, shower doors, glasses, coffee pots, etc.
Although harmless, most people don’t appreciate a white film on these household items. Many customers install a water softener unit. In terms of cleaning hard water spots, there are several cleaning products on the market made specifically for its removal. A "green" alternative is warm vinegar. Soaking in vinegar can help dissolve the spots. Make sure you rinse the items carefully after the vinegar "bath" before using them. This method is less practical for shower doors. In the case of shower doors, prevention is the best medicine. Wipe down the doors with a sponge or towel after every shower.