Moisture and oxidation of the "silver" surface on the back of the glass.
You could try sealing it with varnish or a like product, but I would be
careful about that unless it was a last attempt. If the mirror is just a
mirror, I would replace it. If it were something special, I would contact
the pros. But if it is not something special buy you would like to give a
try at saving it, then try sealing the back from air and moisture.
Dia \'s Muire duit
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