How do I find/delete [greyed out] movies in Sync Movies list for my iphone?

When I plugged in my iPhone to my MacBook and selected the location in finder, I see under Movies several movies (I assume) that I cannot find either in iCloud storage or on my MacBook. At least, I can't find them by searching by name (One's title "Untitled" !). They're greyed out until I make the selection "Sync movies onto iPhone." And when I do this and select one of the movies, the popup warns that syncing "my iPhone" with this Mac will replace media from another library on "my computer." An iPhone can sync with only one computer at a time."

So I've not proceeded, but I'm curious what these few films might be, if they exist at all, and where to find them.