@Anyway, pretty cool!. I did something similar for a MIMOLite controlled water valve, but I created my own device type to add a capability so that my leak detector could open/close it. I added the ALARM capability and then added code to close the valve based on an alarm condition.
If you create your own device type with those capabilities, you can then add a new action for your leak detector in Damage & Danger to “Turn on these alarms” and select your water valve. You don’t need any other SmartApp if you do it this way. It’s all handled through ST’s Dashboard.
Just for fun I took ST’s Zigbee Valve device type and made those changes. I don’t have a valve like yours to test with, but just in case you want to try, here it is: (select Raw to get an easy way to copy/paste the code)
https://github.com/constjs/SmartThings-Devices/blob/master/my_zigbee_valve.device.groovy