wordpress nested loop

Do I see that right, that you are doing a WP_Query for every single post, you want to retrieve? That is highly inefficient, you can use the post__in argument supplied by WP_Query. (http://codex.wordpress.org/Class_Reference/WP_Query#Post_.26_Page_Parameters)

foreach ( $urls as $url ) {
    //echo $url["mymeta_url"];
    foreach ( $url['mymeta_slider'] as $u ) {
        //echo "$u<br> "; shows post id like 67 68 56 etc
        $posts_to_recive[] = $u;
    }
    $my_query = new WP_Query( array( 'post_type' => 'slider', 'post__in' => $posts_to_recive ) );

    if ( $my_query->have_posts() ) {
       while ( $my_query->have_posts() ) {
            $my_query->the_post();
            the_title();
       }
    }

    wp_reset_postdata();
}

And yet this is highly inefficient. Keep the amount of WP_Querys as low as possible.