I've highlighted it in red in the screenshot. It seems to show up almost all the time on my Mac, regardless of what applications I have open. Pressing it does nothing apparent. If I "Customize Control Strip…" in System Preferences, it disappears. It does not appear on Apple's list of touch bar icons.
I'm running macOS 10.12.6 on a 13" MBP.
Best Answer
That looks like it is from BetterTouchTool. You can configure it in the settings for BTT:
https://github.com/fifafu/BetterTouchTool/issues/1033