I had the same problem. Only difference was I am using solarize rather then molokai.
To fix the issue, I have set up an alias in ~/.bashrc:
alias tmux="TERM=screen-256color-bce tmux"
And set up the default-terminal option in ~/.tmux.conf:
set -g default-terminal "xterm"
Lastly, do $ source ~/.bashrc to load new alias.