How to fix leak coming from pressure gauge on water tank?
What you pointed at is the pressure relief valve. Seeing that the pressure is fairly low, it  could either need adjustment or be failing.

The pressure relief valve is basically there to release very high pressure before a pipe/filter/faucet relieves it (eg: blows up).  It's basically just a spring pushing against a piston. 



You can try adjusting it first by turning the end clockwise just until the flow stops. Best to do this at the pump cut-off pressure (which is the highest pressure you'd normally see) -- basically open a tap until the pump runs, wait for the pump to shut off, and then adjust the relief valve.  A lot of the valves actually have a ridged edge you can turn, though yours looks like it probably requires a nut/screw driver or pliers.



If that doesn't work, it is probably just that the valve is failing. Could be a cheap valve or have a manufacturing defect, or could be corrosion. Replacements cost a few dollars at any home improvement or plumbing store and are very easy to install (use teflon tape on the threads).