Run startup command in mosh

This is most not possible this way, because the MOSH itself is already running some command to start its process on the remote server. Also the way you wrote it, it is interpreted as a server name, which is certainly not correct.

As reading through the manual page of mosh(1) (good way to start searching next time), you can specify the command simply as the next argument:

mosh [options] [--] [user@]host [command...]

therefore

$ mosh elisa2 'cat blah'

should just to the job (you don't need to care about tty in MOSH).

Generally, you want to run also the shell, so after the cat, there should be bash too so you will get the prompt:

$ mosh elisa2 "cat blah;bash"