It may be a spot where some of the silver has come off the back of the mirror.

No, nothing you can personally do, however, some glass places can re-silver mirrors.

Thanks. I had a feeling it was something that had to do with the mirror itself.

Right now we have a $40 fix around the mirror, we framed it out with some molding. I'm not going to worry about it, one day hopefully that mirror comes down and will be replaced with something else :)

You might try slipping a piece of aluminum foil behind that section of the mirror, shiny side facing forward.

Move a plant in front of that area. Just kidding! It's probably the silver that has come off the back. I have a mirror like that. I just ignore it.

tape a picture of yourself looking lovely right over it, so even on a bad hair day you see yourself smiling back looking better than ever

I have the exact same thing in the mirror in my bathroom. I just try to ignore it. Only God is Perfect.