Hi,
If you heat the mount arm it will expand more than the remaining bit of bolt as aluminum expands more than steel. It may make it a little easier to remove, looking at the head of the bolt, it looks like you have had a problem with corrosion, the remainder of the bolt may be seized in. Heat on the aluminum will help. I use never seize or copper kote when steel bolts go into aluminum. The lock washer should stop it coming loose.