Can I safely charge a Galaxy S9 with a USB-C MacBook Pro charger? I have a Galaxy S9+ (with "fast charging"), which requires a USB-C charger. I also have a MacBook Pro with a USB-C charger. 

I am curious whether I can (safely) charge my Galaxy S9+ with my MacBook charger. The phone shows that it is charging, and very fast actually.

However, I am not sure whether the charger has too much power and would negatively impact the battery on a short time (perhaps get too hot or even cause physical damage/explode) or long-term (decreased battery health).
All devices (Phones) are 5 Volt technology.

How fast it charges depends on....

Can the charger provide the power the battery is asking for.

Battery is a SMART battery type (it has a chip build in) so it will regulate the charging process safely.