A coordinating conjunction is a word that connects two phrases or clauses together. The two clauses in this sentence that are connected are "Clause enjoyed strawberries but not bananas" and "he decided against a second helping". "So" is the word that connects the two clauses.
The sentence should be "Some of the pond has algae on its surface" since in this case the "its" is possessive (the surface belongs to the pond) no no apostrophe is used.