He sees his own reflections in the dark splotches on your wallpaper, and the shadows cast by the hood lamp isn't helping either. The best thing you can do is try a lighter colored wallpaper or change the incident angle or whatever (I was never a fan of optics in my physics classes) of light. Since you have a hood, this is rather difficult to do, so I guess you can try putting a desk lamp behind the wallpaper to ensure that it is backlit to eliminate reflections.
My red betta stressed me out an exponential amount because he would constantly flare and attack his reflection in dark objects (i.e. filter canister), but my white betta (due to pigment loss from the pet store) never does this. Interestingly enough, my white betta is the dominant one - even though he was in dismal shape when I got him a week ago. Maybe bettas see red as more 'betta-like,' thus causing red bettas to attack their reflections more often?