I’m not sure what to suggest. Are you trying to run both versions at the same time? That’s not a good idea (although I think Apple now claim that multiple versions of Xcode 9 can be run simultaneously). Had you launched Xcode8 before you moved it? I think I read somewhere that you shouldn’t move Xcode after it has been opened.
A quick Google for “xcode launch being updated” finds a few Stack Overflow questions such as https://stackoverflow.com/questions/40327301/xcode-beta-can-not-be-opened-while-it-is-being-updated
Do you still have the App Store version of Xcode installed? Did you try to rename it? Once you try and manually install multiple Xcode versions it is much better to just remove the App Store version. The App Store will keep trying to update that version, which defeats the purpose of managing versions yourself.