Never heard it used that way, though I've heard of Crypto-Calvinists in Lutheran circles, so I suppose that makes sense.. My name on this forum when I first joined, almost a decade ago, was CryptoLutheran. I lost my contact info about a year after that, so I had to make a new account (2011ish?). I chose my current name, ViaCrucis when I did. In order to let people know I was still me, I continued to sign off as CryptoLutheran.
As for why I originally chose CryptoLutheran, I was raised Evangelical/Pentecostal, and in my late teen years and into my early 20's I went through a major period of religious investigation and introspection as to what I believe. The short of that is I ended up finding myself drawn to the Lutheran tradition, but I didn't fully commit to becoming Lutheran for several more years. I was also concerned with how friends and family would act, and so I saw myself something as a hidden Lutheran within Evangelicalism, or a crypto-Lutheran.
That is no longer the case, but the name has stuck, and at this point signing off on my posts the way I do has become an old habit.
-CryptoLutheran