At the time of this writing, the 2018 World Cup is happening in Russia with games being played in several cities including Moscow. The US government has issued travel advisories based on the fact that Americans are often subject to harassment by officials as well as locals. The warning is also due to the fact that large crowds are often targets for international terrorism.
While it is unlikely that you will be detained just for arriving or being in Moscow, certain rules must be followed like not exploring restricted areas. Generally speaking, Moscow and Russia as a whole are safe areas for the well-behaved and documented tourist. It is advisable to use common sense and stay away from potential trouble.