If you are looking for a way to get paint off a mirror, the good news is that it can usually be done fairly easily and without damage. A mirror has a slick, shiny surface so it is already not a great surface for paint to stick to for a long time without easily scratching or scraping away.
You can get paint off a mirror by using rubbing alcohol or acetone fingernail polish remover applied to a cotton pad or cloth. It is best to let the solution sit on the paint for about 10 minutes before you wipe it away.
After all the paint has been removed from the mirror, you will need to use a glass cleaner to get the mirror to look clear again without any leftover residue. Luckily, a mirror is a fairly easy surface to remove paint from but depending on how long it has been there, it can take some time to soak before rubbing away.