getting custom post by tag

Try this code as by using tag_slug__in, not tag.

$search_videos = array(
    'posts_per_page' => -1,
    'post_type' => 'videos',
    'tag_slug__in' => $search_query
    );  

    $new_query = new WP_Query( $search_videos );
    $search_videos_posts = $new_query->posts;