The solution is simple; just type the following in your terminal (adjust for your current version of Emacs):

tic -o ~/.terminfo /usr/local/Cellar/emacs-mac/emacs-24.5-z-mac-5.8/share/emacs/24.5/etc/e/eterm-color.ti


This solution is a slight modification of the answer found on Stack Overflow.

The reason this works is because emacs-mac, the installation of Emacs recommended in the Spacemacs Docs does not include eterm-color terminfo.

Both my color theme and zsh-syntax-highlighting were producing the escape characters. I tried using term, ansi-term, and multi-term and they all produced the same characters. The worst was that every line started with 4m.

With this simple one-liner, my Spacemacs terminal emulator now works as expected.