In the JuiceSSH settings, you'll see an entry for Mosh Server Command which is set to the following by default:
mosh-server new -l LANG=en_US.UTF-8
If you specify the port number you require in this command, the mosh-client built into JuiceSSH will auto detect the server port when it connects.
For example if you wanted port 60500 you could set it to:
mosh-server new -l LANG=en_US.UTF-8 -p 60500
Currently this is a global setting, not per connection which is not ideal if you plan on using different ports for different connections.
We are planning on making this a connection specific setting in a future release.