What is this on the aft end of this GEnx engine?
The pipe is the center vent tube which vents air from the oil sump. 

From airliners.net (User MD11Engineer):


  All gas turbine engines use pressurized seals for their shaft bearing compartments to keep the oil in. The result is that the scavenge oil returning to the oil tank via the scavenge pump, chip detectors and scavenge filter contains a lot of air, it is more a foam than a liquid.
  
  Engines use a centrifugal oil / air separator in the circuit to remove the air from the oil and to vent the system to the ambient to avoid overpressure.
  
  PW and RR use centrifuges attached to their main gear box, which vent overboard through a hole in the cowling.
  
  GE and CFM use the hollow N1 shaft as a centrifuge. The heavier oil sticks to the inside of the shaft and gets returned into the circuit via holes, while the lighter air stays in the center and gets blown out through the pipe in the back.
  
  The air blown out is not completely free of oil, so when the engine has just been shut down, you´ll see some oil vapor coming out from this pipe.




Source: www.airliners.net

Similar vent is found in other GE engines, like the GE90.



Source: airwaysnews.com

The system looks like this:



Source: CFM56-3 Training Manual, thanks to mins. 

This is the actual part in GE90-100:



Source:standardaerocomponents.com