For security reasons, you can not reattach, see https://github.com/keithw/mosh/issues/394
To kill the detached session, use the PID number displayed in that message (that’s the ‘XXXX’ part.) For example, if you see —
Mosh: You have a detached Mosh session on this server (mosh [12345]).
And can run this command:
kill 12345
Also, to close all mosh connections you can:
kill `pidof mosh-server`
Note that if you are currently connected via mosh, this last command will also disconnect you.