The the vSphere Web client form used to modify the NSX exclusion list has 2 panes, the left pane which lists vCenter inventory and right pane which lists NSX Manager current exclusion list as shown in the image below:
The right pane is populated using an API call on the NSX Manager to retrieve the current exclusion list.
If the form’s "OK" button is clicked while the right pane is not yet populated, the form will send an update command to the NSX Manager with an empty list, which the NSX Manager will translate into a request to clear the exclusion list.