Try exiting it and then make sure no tmux process is still running, by for example running ps -aux | grep tmux.
For future reference: the command to kill the tmux daemon (and as a side-effect, all other running tmux processes connected to it) is tmux kill-server (or in tmux, typing <prefix> :kill-server, assuming default keybindings).
For future reference: the command to kill the tmux daemon (and as a side-effect, all other running tmux processes connected to it) is
tmux kill-server(or in tmux, typing <prefix>:kill-server, assuming default keybindings).