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 &amp; 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)