An old anime movie that has ppl going thru a cloud?

ok so there was this movie i watched when i was little and it had a boy with brown hair and a girl with black hair i think and they meet this creepy guy in their window and they go on a blimp thru a storm cloud and are brought to like another world. they get like baths or somethin and are flyin in like giant cotten . later on in the movie some evil brown goop threatens to destroy everyone and everything and the little boy has to try to stop it. confusing i know but its all i can remember. i watched it probably about 6 or 7 years ago. i think. got any ideas?