wait... Isn't cell an alien? Why does it say monster?

Because Cell is a being created by a human, he isn't from outer space.

To be pedantic, anything with Goku or Vegeta in it should be tagged 'alien', but that's beside the point. Cell ain't an alien. :)

He's partly an alien, of three different worlds, even.

he's a bioandroid if you want to qualify him. And as T5J8F8 says he has DNA from 3 alien species: Freezer, Piccolo, Goku