This is a special spacing div that is required for proper vertical synchronization of the grid and the row headers. It is marked with scheduler_default_rowheader which makes it look like row header. In most cases, this is what you want to see (the same background).
It could probably use a special CSS class so you can target it using CSS directly.
But in your case, it might be easier to apply white background to scheduler_default_rowheader_scroll (the row header backgroudn) which will effectively hide this spacing element.