What year is your truck? I can see what looks like an auto locking punger... looks newer than mine
If it's new enough ask Nissan dealer. If not... probably try using one of the rust converters from auto store and then paint. The concern is that it may be rusting from the back?
I had this problem on my last D40, the tub rusts from the inside out. Unfortunately this is just the start, soon the outer panels beside the fuel flap and where the fuel flap would be on the opposite side will start to blister (rust under the paint) and eventually paint will burst. This happens because the area between the inner and outer skins is really well protected and it's a trap for all the stuff off of our fine UK roads. The only way I have been able to prevent this is by getting my hand up in the area between the outer and inner skins and covering it with burst oil. Don't use underseal as all you do is trap in the moisture, oil at least disperses the moisture and this should prevent the inevitable