Meta Slider Lite plugin shortcode in post not working

You need to run the content through do_shortcode() – at the moment you’re just echo’ing the raw post content, so nothing gets parsed.

echo do_shortcode( $content->post_content );

However, you might be better off implementing a proper “loop” and using the_content() template tag – doing so will ensure everything runs “as normal” (firing all the typical hooks and formatting filters), accommodating for any quirks/extra hooks your slider plugin might rely on:

while ( $posts->have_posts() ) {
    $posts->the_post();
    the_content();
}

wp_reset_postdata(); // Restore the global post to the "current" post