Return vs Echo Shortcode

Just define a variable, and concat all html as string and return it.

<?php
function services_shortcode( $atts ) {

    // Attributes
    extract( shortcode_atts(
        array(
            'slug' => '',
        ), $atts )
    );
$html="";
if ( isset( $slug ) ) {
    $args = array( 'post_type' => 'cbd_services', 'name' => $slug ); // -1 Shows ALL Posts
    $loop = new WP_Query( $args );
    while ( $loop->have_posts() ) : $loop->the_post();
    $custom = get_post_custom($post->ID);
    $galleryPhotos = unserialize($custom["gallery_photos"][0]);

    $html.='<div class="accordion closed">
        <h4 class="accordionTitle">'.get_the_title($post->ID).'<span>+</span></h4>
        <div class="accordionContent">'.get_the_content();
        if(!empty($galleryPhotos)){
                foreach($galleryPhotos as $photo){ 
                    $html.='<div class="photoContainer">
                        <a class="fresco" data-fresco-group="'.$slug.'" href="'.$photo['gallery_imagesrc'].'">
                            <div class="hover">#makeitbigger</div><img src="'.get_template_directory_uri().'/thumb.php?src=".urlencode($photo["gallery_imagesrc']).'&w=452&h=275&zc=1" />
                            <span class="photoCaption">'.$photo['gallery_title'].'</span>
                        </a>
                    </div>';
              }
            } 
        $html.='</div>';
    $html.='</div>';
    endwhile; wp_reset_query();
    }

    return $html;
}
add_shortcode( 'service', 'services_shortcode' );