many mac keyboards have a power button. Is there a way to mimic this power button shortcut from an arduino?
There are multiple ways to do that.
You could use an Arduino Leonardo as a keyboard, then send a shortcut command to shut down or sleep the computer. You can edit these from System Preferences>Keyboard>Shorcuts>Services. The keycode 0x66 does not work for mac, but may for other operating systems source
You could write a small program (I'd do it in Python) which communicates with the Arduino over the serial port and shuts down the Mac if the Arduino wants it to.
You could sacrifice an USB keyboard and use a relay to "press" the power key.