Moto 360 Location Symbol
That's the symbol that denotes an app is using your current location. Common apps to use this are the Maps, Lyft, and Weather apps. If it persists for some reason, just try toggling on/off airplane mode or restarting your watch from settings.

I figured this out by opening up Lyft, closing it quickly, and returning to my watchface where that symbol was there for just a second before disappearing.