Mercurial - How do you interpret it / what can it do?

The Child-Thing's mercurial move allows them to change any or all of their looks. They can still be recognized by people who know thwm well, which implies they can't be recognized by those who don't.

As MC, I  have allowed the child-thing to blend into a crowd, but not into a small group of uniformed soldiers.But now I'm second guessing myself. As a fan of the character, should I let this move (which has no direct mechanical benefit) be more useful?

Should they be able to disguise themselves as part of a small select group (uniformed soldiers)? Can they disguise themselves as a specific individual?

How do you interpret this move? How have you used it?

I think you can do it a couple of ways.

First, I don't think the move is intended to be full-on shape-shifting. A strict reading of the move says you may change any or all of your looks, meaning looks associated with your playbook. So you could (for example) go from being a "boy in scrounge wear with an innocent face, bright eyes, and a child's body" to "ambiguous in scrounge wear with a misshapen face, cruel eyes, and a child's body." You will note that there are two elements that don't change - you're always in scrounge wear, and you're always a child.

This is sufficient to fool casual observers or people who don't know you. Say you're pursued through the hold's bustling market. At some point when you manage to break line of sight, you change your looks and Dremmer's goons - who were looking for an innocent-faced boy - totally pass by that weird kid with the misshapen face. Because that's not who they're looking for. The last campaign I ran involved a Child-Thing who (along with several other PCs) effectively got exiled from a community. He (it?) was able to regain entry to that community because no one there really knew him well enough to be able to see the difference, nor did anyone spare yet another urchin a long enough second-look to be able to tell the difference even if they had. And he was feral, so he didn't even need to attract any attention begging for food. He could be socially invisible when he wanted to, which turned out to be pretty rad - mechanical benefit or no.

A looser interpretation of the move would allow you to pick any "look," but at that point if you're talking about mimicking adults I think you're straying pretty far from the central concept of the playbook. And I'd definitely stop short of allowing the move to perfectly mimic a specific person, as IMO there's more to a person's visible identity than their basic look. I don't think perfect visual mimicry is the intent at all.

Also, being able to pick any look raises all kinds of issues with respect to garb - could you just trade your scrounge wear for the Gunlugger's custom homemade armor at a whim? Would that actually give you the equivalent of actual armor? Could you conjure up the skinner's luxe wear? How would you handle gear - especially worn gear? A more open interpretation of the move might be OK, but it's going to take more work on your part as the MC to make calls on a case-by-case basis - and that's generally not good move design (which in turn sort of points me back to the stricter interpretation).

And to answer your last question, if you can't mimic a specific person then blending into a small group of uniformed soldiers is right out, because someone is bound to ask, "Hey, who's the new guy? And why doesn't he have any gear?"