Aristotle did not use the word “essence”, but a clunky relative pronoun with a neuter article: “the what”. In one sense this doesn’t matter. We can just stipulate that “the what” means “essence” sapiens non curat de nominibus. In another sense, this matters a great deal, especially when the question of “the unchangability of essence” comes up. Even after we agree that Aristotle spoke of “essence”, this essence was not unchangeable simply, but only so far as it was formal. Essence requires matter too. At the end of Book Seven of his Metaphysics, Aristotle speaks of an essence that just is form- and he saw that such an essence would be a unchageable without qualification, and therefore would not be natural.
If “essentialism” means the doctrine that there are unchangeable essences to natural things, Aristotle can take this in two ways. If you mean that all essence has form, and so far as it has form it has stability and immutability in some way, then fine. But if you mean that essence can be simply identified with this unchangeable aspect, then you are advocating a sort of bizarre opinion that all things are God and angels- you are in fact, in Aristotle’s mind, denying the very reality of nature. Nature is essentially chageable- this is the very reason why no one before Aristotle though that there could be any real knowledge of it. Aristotle’s solution is not without scandal even today. Consider that he is the first and only person to advocate that truly knowing something does not require that the way we know things be the way they are (all others either deny this, or they learned it from him).