I feel like there was a thread about this not that long ago, but:
* If a certain kind of activity merits a custom move, that should be clear in the fiction; if you are worried you haven't had adequate time to make it clear in the fiction, there's certainly no harm in sharing that the move exists with your players.
* Whether you need to share the entire move with players, or just the trigger conditions, depends entirely on the fiction and the move in question. If it is commonly known what happens to people who 'go exploring in the swamps', then the players might as well see the move. If nobody has ever tried 'opening their brain about Dremmer' before, then I see no reason for the players to know in advance what the consequences might be. They should, however, know that something is up with Dremmer and the psychic maelstrom (see first bullet point.)
* Sometimes you want to let players know a custom move exists so that they are more likely to tailor their play towards it. Sometimes that's a bad idea.
So for your 'dance competition' move -- I would assume that if such a move exists, that means other gangs are already having dance competitions with each other, that this is a part of the setting and that anyone who pays attention to gang politics is aware of the general repercussions of an inter-gang dance battle. This move is a good candidate for total transparency.
However, if you just wrote the move but, in fact, this has never come up in the fiction at all, you should think about a) what that means and b) why you are writing the move. Sometimes you write a custom move because it's clear that something is going to happen, or is already happening regularly, and you want to model its particular consequences. But sometimes you write one because you thought of a cool thing that _could_ happen, maybe only once, almost like a cool plot twist or sudden reveal or whatever. In this case, the custom move is almost a sort of bait -- for yourself, and for the players. But don't just let the move sit there by itself -- it needs to come into the fiction, too, for the bait to be effective, and for the move to feel well-incorporated into the game, instead of a sort of arbitrary one-off (i.e. why are there rules for dance battles but not karaoke competitions?)