Can I safely charge a SGS9 device with a 5.2V MacBook Pro 2018 USB-C charger? Can I use 87W USB-C Power Adapter for MacBook Pro to safely charge a Samsung Galaxy S9 device? I'm curious as on the charger it says the output is 5.2V, while the phone seems to require 5V.

The specs of the Mac's charger can be seen on this image:
According to Wikipedia the USB specification is:


  USB supplies power at 5 V ± 5% to power USB downstream devices. (source)


5% of 5V are 0.25V 

Therefore any USB device have to be able to work correctly as long as the "received" power via USB is within the range of 4.75V to 5.25V.

The Apple USB power supply with 5.2 Volt is within this range.