How can I find this item and either delete it or fix it. I see the resource id, but when I search for that it doesn't show. How can I tell which resource this is?
If you go to Dropbox website or open the folder it syncs to on desktop (assuming you're syncing with a desktop) and go to Apps/Joplin there should be a file <resource id>.md where the id is the one on your screenshot.
If you open the file you should see the original file name - it might help you find the resource.