A clean mirror does not necessarily require the use of a commercial cleaner. There are a number of easier ways which one can use, which are environmentally friendly and often cheaper too.
White vinegar works wonders against mold and mildew stains. You can pour some white vinegar in a spray bottle and spray it onto the stain. Allow it to work for a few minutes. Then rub it in with a soft cloth so as to scrub further in order to help it to remove the stain more effectively.
Using a few drops of lemon extract mixed with some warm water can help to remove most stains, including mold. Just apply onto the mirror with a soft sponge or cloth and rub for a few seconds.
There are some cleaning products which are eco-friendly cleaners since they are manufactured by using ingredients which are not harmful to the environment. If you prefer to use a commercial cleaner, try to read the labels to check what are the constituents of the product and whether it is eco friendly or not. Use reusable cloths, not disposable paper napkins if you want to use a green method.
Make sure to wipe your mirror dry with a soft cloth whenever you notice water residuals or fogging. Also, make it a point to use any one or more of the above methods regularly so as to prevent mold from building up.