My recommendation would be to create you highlight at the intial value of the slider at the same time you plot. Be sure to capture the plot object in a variable.
app.hghlt = plot(app.x(app.x==app.Slider.Value),app.y(app.x==app.Slider.Value));
The in your callback function, just update the XData and YData properties of the object. This is untested. It assumes your slider values can only be values in your x vector, that your x values are strictly increasing or decreasing, and that there are no duplicate values.
app.hghlt.XData = app.x(app.x==app.Slider.Value);
app.hghlt.YData = app.y(app.x==app.Slider.Value);
