Hi,
The mirror you have is a auto dimming item and the fluid in the mirror is settling, hence the line you can see in the glass, it can eventually leak from the glass and if it gets on the paint work can permanetly damage the paint. Very expensive to replace at £300+ from the stealers, though a normal mirror will fit perfectly you will need to connect the mirror heater as the connections are different between the two types of mirrors, or you could simply look for a second hand dimming one on e-bay, which you should be able to get for between £50-70, (this will most likely be a complete mirror for this).