Safi (meaning clean) is a slang-word used in the same way as cool or good. When you are saying something is cool, you don't actually mean that it's cold, it's just a way of saying that something is positive. Do you understand what I mean? :)
"Mmarekani ni safi" translates to, "The American is good". Safi to my understanding has interchangeable meanings such as, good, clean or cool. I hope that this helps!!