Reddit mentions of XLC Alloy Crank Arm - Left, 175mm, Diamond Hole, Silver

Looks like a dog's been chewing on that crank! Cosmetics aside, the taper on the inside of the crank is wrecked, so you need a new crank; pick up a bolt at the same time, it may even be free with the crank. That always happens if the retaining bolt comes out and the crank comes loose from riding, but can also result from forcing it loose without using a proper crank pulling tool.

What you want is something like this: 175mm left crank (not clear from pictures is yours actually is 175, but they commonly are on MTB)- http://www.amazon.com/XLC-Alloy-Crank-Arm-Diamond/dp/B005MHNE72

retaining bolts:
http://www.amazon.com/XLC-Head-Crank-Bolts-15mm/dp/B0043MQV2W

Also, be aware the left hand pedals are reverse threaded- clockwise rotation LOOSENS the pedal spindle. Seen many left cranks wrecked because people tried changing pedals and forced one in wrong, or they made it so tight trying to remove it (via the commonly used counter-clockwise motion) that they stripped the threads.