I nommed these:
Alien is secretly a cat
Cat is secretly an alien

I don’t think they are the same, because the secret identity focus is different in each case.

A fic about how Clark Kent struggles not to reveal that he’s Superman seems to me to have a different focus than a fic where Superman struggles not to reveal that he’s Clark Kent.

But if you prefer to collapse them, it could just be “cat is an alien.”

I’m fine with whatever you decide. Thanks for your modding efforts!