Why won't pics go into hidden folder?

I have pics in the Photos app that I want to put in my hidden folder. My hidden folder is showing in the left pane. I choose all photos and drag them over the top of the hidden folder. My arrow changes to +10 to indicate 10 are going in but they don't. They are still visible with all other photos, not in the hidden folder.

I am trying to do this on my macbook air