Does anyone know what this is? And what could cause it?



Thank you

 

Looks like the reflective coating is separating from the back of the mirror. Being at that low point of the mirror, maybe moisture intrusion? I would have speculated freeze/thaw cycles, but see you’re in MO, and wouldn’t expect freezing yet, unless this has been present awhile?

 

That used to happen back in the early 2000s to BMWs too. they'd turn blue on the bottom 1/2" or around the edges

must be industry common material that breaks down after a while.