Then it is most likely your valve is leaking. Either it is cracked and needs replaced or has a bad diaphragm.

If the grass is in the middle of yard with nothing around it you are most likely looking at a cracked irrigation line. Just dig it up and fix it with elbows and couplers. If it gets worse when u turn on your sprinklers it is a lateral line, if it is bad all the time even if you don't water it is most likely again line so shut off your water to your system before digging it up.