Stretching would be setting the width of the sprite..
So put the origin of the sprite on the left.
Pin the sprite to the player using pin-behaviour.
Give the enemy line-of sight
Stretch (set the width of) the arm when the enemy has line of sight..
If you want the arm to point in the direction of the player, you should set it's angle to something like angle(arm.x,arm.y,player.x,player.y)
If you want it to be better looking, you could change frames of the arm sprite depending on it's width ofcourse..