I saw this movie as a kid and can't remember much of it. I only remember that it's an animated movie about a little kid who opens a door and some like black slime like thing gets out and like swallows everyone and the kid tries to stop it. Can anyone help me out?

Little Nemo: Adventures in Slumberland.

OMG, thank you so much, I've been looking for this for like 7 years, I always did love japanese animation, but at the time I had know idea lol. THANK YOU its funny I just asked posted a question about what movie this was THANK YOU:)