What is this green symbol on the touch bar?

This is no icon or a symbol but rather a hardware error.

The greeninsh flashing rectangle next to the TouchID button has been recognized as sign of a faulty Touch Bar in several cases:

Video:

• Guide to AppleCare for Mac & Overnight Shipping (title is misleading!)

Online Forums:

• Flashing Touch Bar rectangle (text only)
• 2017 MBP touchbar flashing issues (text, the pics and videos linked in the thread don't work for me)
• little white box on touch bar when macbook pro is asleep. anyone know what it is? (the issue was reinterpreted as a "feature": It’s your sleep notification light)