Generally that's a video shot feature if you've turned of visibility (eyeball to the right when you hover)
Have you updated to Wirecast 15.1.1?
What OS (be specific)?
Can you test in a new document?
If you Replace a video shot with visibility turned off to an audio shot it appears gray as well in my testing. The gray can't be changed even if the audio is not muted.
Can you please test with a new document to determine what causes this state for you you?