WP_Query based on URL parameters

Try like this

 $meta_query = array('relation' => 'AND');
 if($queryGenre)
   {
    $query_genre = array(
        'key'     => 'genre',
        'value'   => $queryGenre,
        'compare' => '=',
    ) ;
     array_push($meta_query,$query_genre);
   }
   if($queryMood)
   {
    $query_mood = array(
        'key'     => 'mood',
        'value'   => $queryMood,
        'compare' => '=',
    ) ;
     array_push($meta_query,$query_mood);
   }

and at the end do

array_push($args,$meta_query);