A person successfully and convincingly presenting themselves as a cisgender member of a gender they were not assigned at birth is called "passing". Passing is a complicated and controversial topic, partly because of the issues that it raises for both those who can and cannot pass. It's also a philosophical issue, a safety issue, and a social issue (and a variety of other categories).
I've recently come to realize that I'm genderfluid. No, I'm not some puddle, and you can't use a mop to clean me up (well I suppose you could, but I wouldn't let you). I'm genderfluid: I experience my sense of my own gender as an frequently-changing place somewhere on the gender spectrum; and sometimes multiple places on the spectrum simultaneously; and sometimes I'm not even on the spectrum. Confused? Me, too!