"Even though it can be dangerous if let grow unchecked, a slime is not a monster. By definition, a monster is a sentient creature that naturally harbors hostility towards mankind.
"Despite having the intelligence and the means to communicate, monsters see us as we do with cattle. That's why magical beasts are not considered monsters, because they are capable of understanding human speech and cooperate with us.
"The same applies to Evolved monsters that are called as such simply because they don't resemble animals anymore and because we humans need to put tags on everything.
"The slime is not a monster because it lacks both sentience and hostility. It's a mindless natural scavenger that preys mostly on small animal carcasses. The more they eat the more they grow in size, until they split generating two individuals that are the exact copy of the original one.