The monster seems good, but to me the 7-9 results in your custom move feel more like a miss, not a hit. So, i think it should be something like this:
When you first see the putrescent mutant mass of flesh, roll+WIS. On a hit your mind overcomes the horror in front of you and you may act normally. On 10+, you even got a +1 forward against it.
On a miss choose one:
- You flee from the creature.
- You stand transfixed by its horror.
- You run crazed towards it, weapon swinging indiscriminately.

One other thing occured to  me. For the driver, her car(s) are part of what the character is, so you are not supposed to take them away, even with hard moves (at least not permanently).
For the other characters though, it's just crap, so you can target it with your hard moves.

Am i right?

Hello! I am from Hungary, so DW is getting pretty international :)

We just had our 1st session a couple of weeks ago, so i can tell you what we had problems with. Maybe you would have similar issue maybe you wouldn't. I would actually like to hear how it went the first time for you.

Anyway, there is a possibility from the book, i think to throw the abilities instead of using the standard values. We did that, but i actually wouldn't do it now. Or at least somehow make sure that all characters have at least one 17 (+2). We had one character who had one +1, and all the rest she had 0-s. It was really not fun for her...

While explaining the rules, i forgot to tell the players, that a roll of 6- is always a failure, and it is always bad for them. The text of the moves doesn't make this obvious, so we had some confusion the first time it happened.

Then we had problems with passive checks. I actually asked for help here in this topic, and i think the answers i got were pretty good.

Other than that, just follow the rules (principles, DM moves, etc.) and it should be fun. I actually haven't had so much fun as a DM in years as with this game.

Good luck and let us know how it went!

Thank you both for the insight. Actually, now that it's written down it seems so evident and obvious. But then and there i just didn't know what to do. Guess it comes with practice.

To put it into some context, the sneaking npc was the goblin shaman, trying to get a drop of blood from the wizard character. I added a front to the scenario, that the goblins want to magically bind Florimel in order to use her to help them liberate themselves from slavery. I decided that the ritual needed a drop of blood from the wizard, being her lover once... So, in a way, i think this was announcing future badness, though we didn't explore this further after they killed Florimel very quickly.

Maybe this stealing a drop of blood isn't really necesarry at all, i could've just made a grim portent and have the goblins bind Florimel at some point.

It wasn't my idea. It was on the previous page. As well as that there should be kids, so there are now. I did what you (Philomorph) suggested, name is now not doubled:
http://csomalin.elte.hu/~roland/AW_NPC_v31.htm

However, i have problem with the kids. "Balls is a plain kid female..." just sounds kind of strange to me, but i couldn't make it better. Maybe someone who is a natural speaker of English could figure out a way to fix this.