If you are hooked up to DMX, it won't show. However if you are not using DMX, the reason you see a white box is that the white box is folder 000 and file 000. By default, without DMX attached, the default is for all layers to be full with this file. Somehow you have a higher numbered layer active in mix 2.

My guess also if you are using DMX is that you have layers which are not assigned to DMX which are doing this default. Just turn them off or assign them to DMX also so you can bring them to zero.