Just noticed a busted line below my NX. It’s below the rear driver-side door. Anybody know what it’s to?

 

It appears to be the evaporative emissions pipe/hose that runs from the fuel filler to the charcoal canister that stores fuel vapors.
The point where your pipe is severed should be clipped to the rear fuel tank support strap (refer to image).

 

Wow, thanks! So it looks like the white plastic piece just broke in two; wondering if I could just remove the two halves & take it to a hardware store and replace it, maybe with some hose clamps...

 

Plastic part "A" looks like a connector with internal O ring, similar to those used by Ford. To release the connector, squeeze (or spread) retainer "B" by hand (do not use any tool; they break easily), and slide it off (away from you in the image). Then the hose should separate from the white plastic connector.
You might try a 2-part epoxy adhesive formulated for plastics. Use a razor blade or box cutter to trim back some of the hose at "C". Try to find a piece of hose that fits tightly over the 2 broken sections of plastic pipe. Apply epoxy to both surfaces, then use the purchased section of hose to hold them together until the epoxy sets. No need to remove the hose afterward; it may help to prevent leaks. If this line leaks at low pressure, the check engine light may eventually turn on and a code will be stored indicating failure of the fuel vapor emission system pressure check.

 

Just a guess here but if you are under 80K this may be covered under emissions warranty