get_terms in a taxonomy template

get_terms returns array of terms and not only one term, so you can’t do $terms->slug, because it makes no sense at all…

If you want to display all terms, you’ll have to loop through them:

$terms = get_terms( array(
    'taxonomy' => 'brand',
    'hide_empty' => 0
) );
if ( ! is_wp_error($terms) ) {  // it can return WP_Error
    foreach ( $terms as $term ) {
        echo $term->slug;
    }
}