This is an interesting one, because while I doubt most people even know this movie (also known by the English title Comet in Moominland) exists, it was a significant part of my childhood. In any case, I hope most people at least know about the existence of Moomin as a character and as a franchise, because it’d be kind of difficult to explain everything about it from the very beginning.
The movie is a little over an hour long and it’s not the best anime movie in existence. Having groupwatched it with a few friends (non-Moomin fans) fairly recently, I noticed that the movie is somewhat lazily animated at times. Some of the run cycles look kind of inconsistent and the scene with the carnivorous plant had a lot of reused shots. The backgrounds are also not very detailed, especially compared to some of the more memorable moments from the TV series.
Because of that, it feels kind of contradictory to say that I think the art is still one of the more outstanding aspects of the movie. The watercolour style that is most apparent when the comet itself is in the sky is particularly memorable. The music is also memorable, and I dare say it’s the most consistent aspect across the series of two TV shows and the movie. Even though I cannot in good conscience really recommend the movie to anyone, I do recommend checking out the soundtrack of the series, because I think it’s wonderful.