In my opinion, Moscow is one of the **safest cities in Russia**. 
I'm German and only know a few words of Russian. I've been there 3 times now, first with Russian friends and later by myself. I usually took Yandex taxi whenever possible (there's an App for that), because the rate is fixed before you even enter the car. Alternatively you can settle a price for the ride beforehand, but first, you should look it up and compare. 
I usually mentioned that I "**only have XY Rubels, is it okay?**" so the Taxi driver can't charge you more than that.
I still felt safer over there than wandering around in some areas in Germany Cities. 
Like other answers mentioned, don't wave around expensive stuff, be friendly and open-hearted.