Hello There,
Thanks for writing in! By default, WordPress excerpts which is displayed in your blog index will get the very first 55 words from the post content. And that will only work if you have used the default WordPress editor. When using Visual Composer or Cornerstone, the post contents will be wrapped with formatting shortcode which is now difficult to read for the excerpt and this is why the shortcode is displaying.
To resolve this issue, you will have to insert a manual excerpt for each of your posts. To know more about excerpts and for you have a clear understanding of how it works, please check out this codex:
https://codex.wordpress.org/Excerpt
Hope this helps.