How did mad-eye's eye see through harry's cloak? (harry potter question)?

I don't think being a hallow makes the cloak completely impenetrable. The main benefit to Harry's cloak being a hallow is that it simply works better than normal invisibility cloaks, i.e. it doesn't fade with age like normal demiguise cloaks do. Harry's cloak is a very powerful magical object, but so is Moody's eye. Harry's cloak works better than most invisibility cloaks, but you have to remember that it is just a cloak until it is united with the other two hallows.