There are a few things here if you can clarify!
Are you housing the tetras and the betta together? I've heard tetras can be a little nippy.
Did you put in a leaf from an aquatic plant?
Is there anything in his orginal tank that he could have gotten caught on? Sharp plastic plants, rocks etc..whatever..
Don't put the mirror in anymore until we figure this out. He's already stressed and doesn't need the extra stress from a mirror.
I'd probably do a water change with treated water, keep him warm and alone, some might say add aq salt but I don't do this so someone else can advise..