Hide Menu items base on capability

Create a custom walker. Redefine the method start_el() only:

function start_el( &$output, $item, $depth, $args )
{
    if ( 'Your secret item title' !== $item->title 
        or current_user_can( 'administrator' ) 
    )
    {
        parent::start_el( &$output, $item, $depth, $args );
    }
}

Please mind that this is just pseudo code. I can not give it a test at the moment.