I'm looking for an old, animated movie I saw when I was a kid.

I don't remember a whole lot of it, and unfortunately I don't know when it was made either. I do remember it was a fairly unique movie, though, so hopefully what little I do remember will be enough to help find it.

Things I know for sure: Animated movie, at least 17 years old

Things I'm almost positive about: Animal cast, much like the American Tail o Rescuers movies, took place in the US, possibly New York

Things I'm fairly sure of: Part of the plot had something to do with some sort of inanimate grey [Gray? I can never remember which is the spelling for the color, sorry ><] slime that had covered many animales in the sewers, making them look something like Grimer, near the end of the movie something happened that caused the sun to shine down on the slime, making it harden, crack, and break away. As a side note, I'm partially colorblind, so the slime may have been purple-ish as well.

I haven't seen this movie since I was a little kid, and I've recently gotten a craving to see it again, but I can't for the life of me remember anything más about it :( I hope someone can help, this has been bugging me for a while.