why captain smollet took offense to "Cap'n Silver."

This really depends where in the book you are talking about. I'm going to pick Chapter 20.Captain Silver was a deserter and wants to come back on the ship,

Why, Silver," said the captain, "if you had pleased to be an honest man, you might have been sitting in your galley. It's your own doing. You're either my ship's cook — and then you were treated handsome — or Cap'n Silver, a common mutineer and pirate, and then you can go hang!"

The captain doesn't want to let Silver on board despite his arguments to be let back on. Hope this helps a little.