This confusion of yours is maybe why people should stick to Latin names for grammatical phaenomena.
In your example sentence
Es wird mir geholfen.
es has no function other than filling the Vorfeld, the area before the verb. Therefore, it is an expletive; there only for syntactic reasons. The correct German name for this is Expletiv. If you really wanted to translate it into German, I would use Platzhalter, because it occupies the space that would otherwise be unfilled without carrying a meaning.
However, there are other sentences where es functions as a pronoun.
Es ist ein hübsches Bild.
Es, here, is referring to certain picture known somehow by context or by direct reference. The correct name would be pronoun or Pronomen but if one really wanted to use a German word, I would recommend Stellvertreter, because it substitutes the other word which was considered too long by the writer/speaker.