Sorry, in the meantime I have learned that the orange MIC symbol is normal for MacOS and that you have to grant audio applications access to the microphone so that they can also access core audio.
My problem with the Quad Cortex is not related to GP5. I also have the standalone Guitar Rig 7 application on the MacBook and it gives the same crackling when I select the Quad Cortex as the audio interface.
Why didn’t I think of this before? 
Thanks for asking about other audio applications on the computer. 
I will have a look at the Neural DSP forum. But many thanks for the help.