That's a pressure relief valve that is leaking. It is designed to relieve pressure should the pump not shut off for some reason. Tanks, such as the one you described, have a maximum allowable working pressure. The relief valve is there to keep the tank from being overpressured and rupturing.
Your valve is set to relieve at 75 lbs.
Replace it with a new 75 lb. valve, but first ascertain that your system pressure isn't too high (that the pressure gauge is correct).