The Nissan Path

Hi,
Ive done an engine swap after old injectors caused my yd25ddti engine to fail. Running OK but I have this leftover plug im unsure where it goes? cant recall it being unused prior to the swap but cant for the life of me find where it goes. See the attached pic.
The plug is green and sits near the rear of the fuel pump.
Cheers for any help.

Sorry the Diesels are almost unheard of in the USA and this is where most of our membership and techs hail from.