Well, in this context, it was my response to you saying "mister/miss". I am non-binary, so "Mx" (pronounced "mix" or "mux") is a more appropriate choice.

It also goes at the start of a name, so my full name would be Mx. Hellfire One O'Three (using my username as an example, so as to not give out my real name).

I suppose you could use "mistrum" instead, since it's gramatically correct; but this isn'y widely used.