alternatively, you can say yes, it will display new layers & right click, select all, right click, reconcile and the error will go away.
the intent is to warn you that new layers exist in xrefs and stuff might be showing up in viewports that you weren't aware of. If this happens on a daily basis and your ok with it, turn it off. But if you want the functionality, reconcile the layers and the message will go away until new layers are added.
