Go to the film's title page and hover over the + for Add to Watchlist, waif for the drop-down menu, scroll to find the list name, and uncheck the selection.
-or-
Go to the list, select Edit from the drop-down menu (3 dots, to the right of the list name), and find the title on the list. (If you have more than 250 titles on the list, you may need to advance to another page, or you can reverse the list order if you think it was added recently or long ago). Once found, check the box to the left of the title, and scroll back up to the top of the page to click the Delete link.
If you're using an app instead, it would have been better to select that as the category for your post. But now just post here which app, and we'll see what to do next.
