Did you manually move the files? Or did you have Steam move the game?

I suggest you do a clean install of the game. It sounds like it didn't move properly. Moving the files manually might have messed up something. Just make sure to back up any saves you have.- Uninstall the game through Steam
- Manually delete all these files and folders, if still there:...\Steam\steamapps\common\Cities_Skylines\
...\Steam\SteamApps\workshop\content\255710\
...\AppData\Local\Colossal Order\Cities_Skylines
(first move the /saves/ folder elsewhere to keep your save games)
(To see the AppData folder you'll need to enable View Hidden Items in File Explorer)

Yes, you would need to reinstall the game. But the game isn't working right now for you, likely because you moved some files manually. Steam doesn't do well with that. So my best advice to fix it is a clean install of the game.