I have no idea what that is behind your mirror,
but let’s get rid of it.
I had to pull off some mirrors in a few apartments lately, you’d be surprised what I’ve seen. Several of the mirrors had large holes cut into the wall, perhaps for plumbing repairs. This one was a real mystery to me. There was some mold growth on the wall, no moisture was detected in the wall.
We did clean up the mess before returning the mirror back though.
Specialists in Dry Rot and Water Damage Repairs