How can I find this all pass filter's phase? How can I find this all pass filter's phase? 

$$H(e^{jw}) =\frac{−a* + e^{-jw}}{1 - a(e^{-jw})}$$
where $a = ρe^{jϴ}$ is the pole.

Then I want to find group delay, I know that the group delay is the derivative of the phase with respect to w but i don't know how to find the phase
phase of a filter is the angle (or "argument") of the complex number that is $H(e^{j\omega})$ at each value of $\omega$ .  you may need to "unwrap" the phase by adding or subtracting integer multiples of $2 \pi$ .

BTW, to post math to these pages, surround the $\LaTeX$ expression with single dollar signs for inline math and double dollar signs for math expressions on their own line.  if you need a quick guide for LaTeX, my suggestion is the one that lives at Wikipedia: https://en.wikipedia.org/wiki/Wikipedia:Math .  i'm still learning how to do other markup here.