Hi,
The HP Color Laser MFP 179fnw is mentioned in About AirPrint - Apple Support as a printer supporting the driverless AirPrint technology.
>I can't find the printer WiFi password to connect to it
Normally, you would use a Wi-Fi router (infrastructure mode). The printer would be connected to the router, either wirelessly or by Ethernet (to one of the LAN ports of the router). If the printer is wirelessly connected, you would need the password of the router's Wi-Fi network. Alternatively, you could use WPS (if supported by the router) when connecting the printer.
http://h10032.www1.hp.com/ctg/Manual/c06266695.pdf#G6.5719498
The Mac and the iPhone would be connected to the same network.
If necessary, try resetting the printing system and re-adding the printer (as an AirPrint printer).
Reset the Mac printing system to solve a problem - Apple Support
Add a printer on Mac - Apple Support
If there is a problem with AirPrint, you could try the following power-cycling sequence:
Power off the router, the printer, the computer, and all other devices (iPhones, et cetera). Wait two to three minutes. Power on the router. Wait two to three minutes. Power on the printer. Wait a minute or two. Power on the computer. Finally, power on all other devices.
Use AirPrint to print from your iPhone, iPad, or iPod touch - Apple Support