Is it really necessary to wipe the powder off of dried kombu?
The consequences are a relatively trivial loss in flavor-enhancing compounds (the glutamates). There's no reason to waste water removing it.

I vaguely recall some books like Gaku Homma's Japanese Country Cooking suggested you could wipe off the kombu with a wet paper towel if you like, but that's not really necessary, or in my experience, even remotely common.