How to calculate phase for discrete time complex valued signal
For a complex number expressed in the polar form, $x = r e^{j\phi}$ , the magnitude is $$|x| = |r e^{j\phi}| = |r| |e^{j\phi}| = |r|$$ where $r$ is a real number and the phase is $\phi$ in radians.

When the complex number is expressed in rectangular form $$x = a + j b =r \cos(\phi) + j r \sin(\phi)$$ then the angle is expressed as $\phi = \arctan(b/a)$

For a sequence of complex numbers $$x(n) = e^{(-0.1+0.3j)n}$$ for each n,
one can find its magnitude and phase once writing it in the polar or rectangular form where the polar form is $x[n] = ( e^{-0.1 n}) (e^{j 0.3 n})$. 

Then its magnitude is $|x[n]| = |e^{-0.1 n}| |e^{j 0.3 n}| = e^{-0.1 n}$ and its phase is $\phi = 0.3 n$ radians for each n.

Note that principal value of the phase is the angle beetween $-\pi$ and $\pi$, any value other than this range can be equivalently mapped into it.