What region / country are you in, and why would any alleged "hacker" Install MS Excel, never mind that installing a user app, as a system app, couldn't happen without Root.
Thing is, that the only ways to make a user app, a system app, is with physical access to the phone, AND gaining the equivalent of Windows Administrator Rights, which requires Root access. Even then, if it's not a System app baked into the Firmware by Samsung, you should be able to disable it.
Hi amélie, can you please provide us the model of your device?
Also, if you think your phone has been hacked, the very first thing you should do is perform a factory data reset. This will clear all of your personal data off of your phone and kick out the hacker.
Once the phone has been reset, you should change the password for your Samsung and Google account, along with any other account you had on the phone, like bank accounts, etc. If any of your accounts have security questions, changing them may be a good idea. On top of this, you should turn on two-step verification for any accounts that have this option, including your Samsung account.
