Fountain of Knowledge

The three grammatical genders are male, female and neuter; my guess would be that, since the catalyst has no analogous gender construction, we would think of it as neuter.

For what it's worth, neuter as a gender in English is rarely conceptualized that way; we have he and she for animate objects and only it for inanimate. In pronouns, neuter and gender-not-specified all take "his" as possessive, but the meaning of that has sort of been lost. In other languages, however, third genders are common. For instance, in Russian, there is male, female and neuter, and the neuter has its own declension and usage throughout pronoun usage. ("He" is on, "she" is ona, "neuter" is ono.)

I think for your world-building on this, you should consider where the story is taking place. If it's in a country whose language already accounts for the possibility of a neuter gender, they would more than likely use that existing framework to account for the third gender of your aliens. In an English-speaking country, there would probably be much more difficulty. In fact, there would likely be great debates among grammarians, in legislatures, and in the media over what to do! My guess is that the average person would just say "it," or "the third thing, you know," and shift uncomfortably, while more highly educated persons, especially those invested in proper grammar, would make lengthy pleas for the return of a neuter pronoun to Germanic languages. Unless the aliens have been around for a full human generation (since it takes about that long for new grammatical usages to be fully institutionalized in a population), I think there wouldn't be anything standard, though there might be movements toward standardization. That could be an interesting plot point.

Does that help at all? Let me know if you have any questions...

Lorien