It will absolutely move, depending on the weight of the tool you have in the head. Especially when water is in that hose. There’s no electronic lock/break and the drive is 1:1 at the motor and the threaded rod is not that high of a pitch. Once the Delrin is good and broken in and that rod is coated with it, it will turn real smooth and will drop on a power loss.
I actually measured the drop on my machine of 1-5 mm, even without water in the head. It might actually run all the way down with water in there, but I’m not ready to fill the hose with water yet 
We’ve been talking about this subject in another thread here: Documentation for using the web app -- setup