In the Facebook post that sparked this, Mysterious Emotion X is clearly described in terms of other-regulation: a "status slapdown emotion". Shame and embarrassment, chronic and low-grade or otherwise, are directed at self-regulation, so they aren't a good fit. Envy (and "a sense that someone else has something that I deserve more", which sounds to me like resentment) is specifically excluded, so it's not that either.

I'm pretty skeptical of the microaggression model too, but this isn't the place to be talking about that, if there exists such a place.