Coolant leak around oil cooler connection

Hi Folks,
I have some coolant leaking out of the oil cooler connector at the top of my radiator.

It looks like it's coming from the "back-most" part of the connector - ie. the stud.

I can see the thin nut on the stud itself - can someone tell me, what does this tighten against?

I haven't touched it yet, but might give it a small tweak up, but wary of threading it.

I'm reluctant to use radiator stop-leak. Not a big fan of the stuff

If anyone has overcome this before and has some tips, would be greatly appreciated

Cheers

PS. Apologies for the poor photo, my phone's camera has stopped focussing.

Is it leaking at that nut or the pjp goin to it.it cud be loose.the washer in the pipe could be perish and seeping thru

Not sure what the pjp is sorry. I wouldn't have thought it would be the washer in the pipe, as wouldn't it leak oil rather than coolant if that was shot?

Pipe