The modulus of $ z $ is:
$$ |z| = 1$$To find modulus of a complex number $ z = a + bi $ we use formula:
$$ |z| = \sqrt{a^2 + b^2} $$In this example we have $ a = - \frac{\sqrt{ 2 }}{ 2 } $ and $ b = - \frac{\sqrt{ 2 }}{ 2 } $ so:
$$ \begin{aligned}|z| &= \sqrt{ \left(- \frac{\sqrt{ 2 }}{ 2 }\right)^2 + \left(- \frac{\sqrt{ 2 }}{ 2 }\right)^2 } \\[1 em]|z| &= \sqrt{ \frac{ 1 }{ 2 } + \frac{ 1 }{ 2 } } \\[1 em]|z| &= \sqrt{ 1 } \\[1 em]|z| &= 1 \\[1 em] \end{aligned} $$