+1 on the above. ^ but not neccessarily one particular plug, could be one, could four plugs. Someone didn't tighten one down fully I suspect.
That hole is a weep hole should water get into the spark plug wells. Water runs to the left side of the bike(due to the side stand) and out that hole as its designed to.
Looking at the color of it, it appears a loose plug, along with moisture, unbruned fuel, etc, is weeping out of the hole... I would definitly address it,
you don't want a spark plug hole getting wallowed out should one be loose...
Time to loosen some bolts, get to the plugs and either just tighten, or replace them while your there... About 20 minutes with the correct tools..
Be very gentle with the spark plug caps, they tend to stick, the wire sometimes separates from the cap itself causing problems later... Dilectric grease helps with the re-assembly and later when pulling the caps, helps keep moisture out too...