Cannot remove slide number box that only shows up in preview

The "%slidenum%" text is a reference to a variable called slidenum. 

If you're not seeing it in Slide View but it shows up when previewing, it's probably on the Master layout(s). So check that, and look for any layers on the Master, because it could be there. 

Here's more info about Masters: https://community.articulate.com/series/articulate-storyline-360/articles/articulate-storyline-360-user-guide-how-to-use-slide-masters