why cant I delete in elements organizer

Not moved from another location. I dragged them from my desktop into the element organiser for a practice edit. Now I want to remove them so I can work on a new project. There’s 3 short videos and a few ( don’t know where they came from) white folder icons with question marks in the folder. I need to drag new videos for another project and the elements organiser folder is already cluttered. 

Were they moved from their original locations?

Are they on an external drive that doesn’t have a fixed drive letter?