No matter how wax got on your mirror, you can remove it without scratching the mirror. You may be tempted to just start scraping the wax, but this is tedious and not the most effective way to get the wax off completely---plus you may damage the mirror by scraping hardened wax. The key to wax removal is to soften it.
Turn a hair dryer on high and aim it towards the wax, holding it about 6 inches away from the mirror. As the wax softens, use your fingers to pull the wax off the mirror's surface, or carefully scrape it off using a plastic blade.
Put away the hair dryer. Extinguish open flames (in other words, do not smoke and if candles are burning nearby, blow them out). Spray the wax residue with WD-40.
Let the WD-40 sit on the wax for one minute. It will make the wax easier to remove.
Scrape off (gently so you don't scratch the mirror) more of the wax residue and then any remaining wax with more WD-40. Repeat this until all the wax is gone.
Wet a soft cloth in clean water. Wipe the WD-40 residue off the mirror and then dry it with a towel.
If you only have a small layer of wax on the mirror, skip the first step and just spray the WD-40 on the mirror before scraping. Any dull plastic blade will work for scraping the wax---you could use a razor blade, but you will increase the chance of scratching the mirror if you do.