Exclude categories from wp_list_categories by category slug

I would suggest a cleaner approach would be to implement an exclude_slugs argument for the function, then you can just use the following in your template code:

wp_list_categories([
    'exclude_slugs' => [ 'fp-feature', 'fp-aside-feature' ],

    // Other arguments
]);

Here’s the filter, pop it in your functions.php:

function wpse_217847_list_terms_exclusions( $query, $args ) {
    if ( ! empty( $args['exclude_slugs'] ) ) {
        if ( ! is_array( $slugs = $args['exclude_slugs'] ) )
            $slugs = array_map( 'trim', explode( ',', $slugs ) );

        $slugs  = array_map( 'esc_sql', $slugs );
        $slugs  = implode( '","', $slugs );
        $query .= sprintf( ' AND t.slug NOT IN ("%s")', $slugs );
    }

    return $query;
}

add_filter( 'list_terms_exclusions', 'wpse_217847_list_terms_exclusions', 10, 2 );

Bonus points: no additional db queries needed to first get the slug => ID translations.