I think it was just to put emphasis on elvis. A little uniform separation. That white border was in front of the guitar player also.
Gotta admit, gigging with a good Elvis cover band would be super fun, I would stand in a box. {} Don't know who he is either but based on his instrument choice and bass face alone, looks like he was Killin.
Seriously? How else would anyone notice the bass player stood in a box behind someone dressed like that??