The plug is your coolant temperature sensor. It controls the factory temp gauge.
If you are getting a leak in that area, it is most likely a leak at the manifold. Have you tightened all the manifold bolts? They are notorious for coming loose, especially the ones near the firewall, near where the temp sensor is.
Perhaps you can carefully bend the valve cover outward, away from the valve spring, just enough to clear without screwing with the bolt hole locations.
Now I lay me down to sleep.
I pray the Lord to fix my Jeep.
If I should die before I wake
Roll this POS into a lake!