When you show a layer in Articulate Storyline, you have the option to hide objects on the base layer. Mark the Hide objects on base layer property to hide all base layer objects, or click the eye icon in the timeline to hide individual base layer objects.
However, when you add a video to the base layer of a slide using the Video from Website option, you may find that it doesn't hide as expected when you show a layer.
This is a known issue. You can correct it by inserting the video as a web object instead.