What is this grey git icon?

Looks like a folder inside of a folder - or maybe a lock? The file cannot be opened and none of the content appears to be there.

One thing I do know is that this folder is a repo itself, inside of a repo

Considering it is a submodule, one way for you to see its content is to:

• clone that gitHub repo locally

• do a:

  git submodule update --init
  

But you should be able to click on it and browse its content directly from the web, if it was a repo hosted by GitHub.
See an example in my project https://github.com/VonC/compileEverything, where the submodule SemanticUI is directly browsable.