Best way to describe it is to show you a picture.
Almost looks like the wall has been photoshopped in.
What material are you using or did you get it from a tower of some sort by chance?
Try selecting the wall and hit F4. scroll to one of the lighting properties and tinker by removing stuff like ‘Cast Shadows’ and see if it does anything. Don’t remember the exact name of them all. May edit later when I get more time to check.