One of the sprinkler zones was turned on as part of the daily schedule. App shows it was turned off after the schedule but I just so happened to step out after 2 hours and noticed that the sprinkler was still on. App still shows that it is stopped. Not sure how do I stop the sprinkler. Any help would be greatly appreciated!
I suggest first unplugging the controller and seeing if the zone turns off. If it does, something might be up with the controller. If it does not, it seems something might be up with the valve which likely means it could use some cleaning as it may have gunk in it preventing the valve from closing.
First and easiest is like @Thomas_Lerman said, unplug the controller to make sure there isn’t a rogue power bleed going to the valve.
But, chances are that zone valve has a torn diaphragm inside it that needs to be replaced, or possibly some debris that needs to be cleaned out. Lesser chance, but still a possibility is that the solenoid could be stuck, or a seal failed on it. I just had to replace one in my yard that had a failed plunger.