The resource I added was one that actually can't be added through Appery's external resources bcoz it is a script tag with attributes. However, this case represents any resource that is mistyped, and should be removable.
I tried to add and remove another resource, as suggested in an old post here, but to no avail. I don't want to manually remove from source file each time I publish.
This is the external resource list after deleting:
This is the leftover: