Remove sudomain from Google search console but keep indexing it.

Hi all, I have a quick question:
We have a commercial site which we added to search console. All is working fine, however one of our web-apps which lives on a subdomain is being treated by Google as if it were a part of our site. This app is not mobile-friendly, so it is hurting our score. How can I tell Google to treat this subdomain as a separate site? I have no control over this app nor the dns, so adding it as a separate property is not really an option. Additional complexity: the subdomain should still be indexed!

TLDR; How do I tell Google the content on a subdomain is not part of my main site?