how do you change the motd, or remove it entirely?

its annoying having

Welcome to fish, the friendly interactive shell Type help for instructions on how to use fish

come up all the time

set -x fish_greeting ""

You probably meant set -U because you probably don't want to export fish_greeting to child processes. fish_greeting is already universal though and you don't need the empty string to unset it. This is enough:

    set fish_greeting

set -e fish_greeting

-e to erase