I believe that slang is where a language starts to develop itself, independently and deliberately. The small danger here is that "slang" cannot be universally understood (which is why I advise learners not to use it), and is often used to mark cultural territories.
Just saying that something is "slang" is a very misleading comment about when, where, how or even IF a phrase could be understood. Even if it is understood, it could be interpreted as invasive if you're not actually from that "group".
Aside from that, as a concept it's very poorly-defined.