I am certainly not an expert in this, but I assume that the crack is in the glass and not in the mirror finish on the other side. I also assume that this mirror was hung up with an adhesive instead of a different type of mount, and it is mounted on sheet rock.
The problem I suspect you are having is your wall is shifting, sagging or tearing on the other side of the mirror and the adhesive is transferring that stress to the glass mirror that is acting as sheer wall. And, the mirror is sheering in response to the stress.
If this is the case, most likely, you'll have the mirror crack as the wall would have it for as long as the wall is shifting.
The only way to stop a crack is to drill a hole in the material at the leading point of the crack to relieve the stress. I've never tried this with glass, but it works well with metal. The trick is to find the leading point of the crack – not too far ahead of the crack, and not behind the leading point.
You can't use a regular drill for this, you'll have to use a diamond tip bit meant for drilling in glass or ceramic.
I would call a glass shop and see what they suggest. You may have to take the mirror off the wall just for safety reasons. If it comes to that, I'd tape the entire surface to reduce shattering and splintering before you try removing it from “gawd knows what” adheres it to the wall.