Review Pre-Screen

That's the Play button to start the course. It's not controlled by Storyline. It appears because the browser requires the user to click/tap a Play button to allow audio and video to play. 

I've seen numerous posts about this in the Forum. It typically appears if the first slide has audio or video content. So the fix is to add a slide before that content. That new first slide could just be blank, with a very short timeline, and a trigger to advance automatically to the next slide. All that would basically make it invisible to the user. (You'd also want to delete it from the Menu if you're using the built-in Menu.)