It does look broken/twisted. If the arm seems to be causing discomfort you take them to a vet as they should be able to amputate so the arm grows back straight, or you can at least get their opinion. The arm will heal regardless, but if not amputated it may just heal in this position and always look like it is waving at you, haha.
Is there another axolotl in the tank that could have caused this?