I've never had a problem cleaning out hydration packs. Usually, I'll just put some bleach & water in it, and rinse it out good. If it is really bad inside, I would use something like Steri Clean, which is a pink powder cleaner that is used when cleaning bottles for home brewing. It'll clean up any crud built up on the inside, plus it is a sterilizer, so it'll be safe to use afterwards. Just have to rinse it out really well.