Worldlearner has spoken about the meaning, so I’ll just go over the structure. The brown blotches: Subject (of the main clause) of the benevolent skin cancer: Prepositional phrase qualifying “blotches” (that) the sun brings: relative clause qualifying “cancer” (or possibly “blotches”). from its reflection: prepositional phrase qualifying “brings” on the tropic sea: prepositional phrase qualifying the reflection (its location) were on his cheeks: This is the predicate of the main clause. So essentially, the main clause is “The brown blotches were on his cheeks”, with everything else giving more details (important details, to be sure). Hemingway is famous for this style of writing — which is more like painting a picture with words, rather than describing actions. His style is special, and not everyone should attempt to imitate it. Having not read that story in a long while, I have no idea how the cancer could have been “benevolent”. Perhaps he was being ironic, or maybe that word had the meaning of “benign” (a benign cancer is one that is fairly harmless).

This is very hard for even native English speakers to understand. The word 'benevolent' doesn't seem to fit in the sentence. Some people have theorized that this might mean that the cancer will eventually put the man out of his misery instead of him having to further face the ravages of old age. However, these are just theories. Also, if a similar student was written by a student in an English class, it is likely that their teacher would mark off points for it being nonsensical.

*similar sentence