The glass in some car door mirrors is made from a thin film and can be easily scratched. If your door mirrors are covered in frost, it’s best to avoid scrapping them as it may result in damage.
The environmentally friendly way to melt the frost on your door mirror glass is to poor water over them. Ensure the water is lukewarm and not too hot and as soon as you’ve done melting the ice, use a soft microfibre cloth to dry the mirror to avoid it freezing again.
Alternatively, if you do not have a can of deicer at hand, make your own. If you have some rubbing alcohol at hand, mix 2 parts alcohol to 1 part water into a spray bottle. You can also use this on your windows and windscreen.