That or he watched the anime, or he just spends a lot of time on wikipedia researching random stuff.
Source
This comic was published the same year Cassandra Cain was introduced, so I choose to believe he knows Sailor Moon because he watched it with her.
The out-of-universe explanation is that the writer asked a friend to give him a name associated with Mars, and said friend was a bit of a smartass. But I choose to believe that this was the in-universe explanation. XD