That "metal arm" does not protect the master cylinder, it is the master cylinder. It is not wise to attach the mount there as, if you drop the bike (always a possibility for a new rider), and the phone puts pressure on this you will end up with no brake when it fractures. Somewhat inconvenient.
As a grumpy old git I would say, especially at your stage, forget the phone and concentrate on riding the bike.
Last edited by PCV; 07-20-2019 at 02:26 AM.