Hi there
Do you have a web development team or a web developer? What I would do is pass this notification over to them, along with your notifications from Moz, and see if they have a means to correct these issues. I am assuming that Google passed along resources in their notification; I would look into those and see what your options are.
If you do not have a web development team, I would check out the Recommended List to find a company that does web development as well as SEO that can assist in this. What it sounds like to me is that you are linking off to an app with a subdomain and it's creating a different user experience than the one generated by your website.
If I were you, I would find a suitable blogging platform that you can bring your sharing capabilities onto, and create a consistent and seamless experience for your users. Two questions:
- Is your subdomain blog.domain.com? Or is it named differently?
- Do you have your blog posts on your website and copied word for word on your subdomain?
Here are a couple of more resources to review with your team:
App Indexing for Google Search Overview What is App Indexing?
App Indexing for Google Search Technical Details Enabling Deep Links for App Content
Let me know if any of this helps or if you have any more comments - good luck!