Restart WSJTx.  If you do not hear 7O73T, there should be <. . .> on the left side of stations CALLING him since you have not seen him yourself.  

If you type in S20MUJ in the DX CALL box that will show up in the next cycle.  And, if you should hear the 7O73T, his call will not show up, but instead the S20MUJ will.  

Now shut down WSJTx again and put in 7O73T in your DX Call Box and all will be corrected.  

So it appears that S20MUJ is not a pirate, but a bad hash that will show up, especially in a "compound" call from the DXP as just <>.  This will only happen on DXP with a non-standard call.  I noticed that the S20MUJ was always where 7O73T was, but never put them together as in reality, the same station.

In other words, "A hash crash" has occurred as K1JT has said along could happen.