"EMACS" string in tmux/screen powerline prompt if started from zsh

Since I use zsh with powerline there is a weird issue I have when starting tmux (or screen before the switch to tmux): there is a string at the beginning of my command prompt saying “EMACS”, with green background (solarized light color scheme, in case that is important):

I have no idea where that comes from. I do not have emacs installed, I prefer to use vi. Does anyone has any idea where that comes from? And how to get rid of it or change it to “tmux”?

The string does not come up when I start tmux from within a clean bash.