I have always just stuck our Nylon halters in the washers :p But we only do it about once or twice a -year,- so if yours are dirty every week, you could really beat up your washer. Our horses go out without halters, and IMHO, that's how it should be. Leaving halters on encourages biting at the face, tearing up of halters, and rubbed up noses. Give em a good clean once in a blue moon to give em a refresher, and you've got halters that last a lifetime :)
I HAVE washed mine in my machine, but it's kinda hard on the washer. Is this a nylon halter? You can use a mild soap, and let it soak, then spray with a nozzle to get All of the soap out. Then, just hang it to dry.
I love to periodically clean up my halters and leads. It's kinda like cleaning and sweeping out your car. You wonder why you don't do it more often! =b