2006 6.0 leaking from back of the engine , clear fluid

Likely to have an injector dumping fuel into that cylinder.

If it is diesel, then a new injector most likely.

If it is dripping, then smell it, run it in your fingers to see how oily it is or isn't, etc. You could even get a paper towel to soak up some and see if it will burn.

You are in the best position to know for sure what it is.

What led me to suggest the injector is how the area on the "cab side" of the exhaust manifold flange looks dry, but the engine side looks wet. It may just be the picture quality. If there is excess fuel in the exhaust manifold, your engine would have some exhaust smoke for sure, so let us know about that if you do have exhaust smoke.

Regardless, checking that fuel rail plug on the end of the head is a great suggestion for sure. Once you get a leak, the air across the engine usually pushes it back towards the bed of the truck, but as I am sure you know, a leak can flow into strange places.....as an example, seems like almost all oil leaks end up on the bell housing - even the glow plug leaks and cam/crank sensor leaks.