Answer
You would tell the Martian that their "discovery" is already known to us:
$M(x)=\displaystyle \frac{x^{2}}{2}-x+C$
Work Step by Step
If you find one antiderivative of $f(x)=x-1$ (say that it is $F(x)$ ), you've found them all, because any antiderivative of f has the form
$F(x)+C.$
It just so happens that $F(x)=\displaystyle \frac{x^{2}}{2}-x$
is an antiderivative, so $M(x)$ has the form
$M(x)=\displaystyle \frac{x^{2}}{2}-x+C$
