Run this on terminal to add to your .tmux.conf:
cat - <<-EOF
#bind m to Enable/Disable mouse (change to whatever you fancy)
bind m if-shell 'tmux show-options -vg mouse | grep on' \
"set -g mouse off \; display \"Mouse is off\"" \
"display \"Mouse is on\" \; set -g mouse on"
EOF >> ~/.tmux.conf