Barnacles grow on ships due to inactivity.
So when someone at your office makes a comment implying that barnacles might be forming on your ass, they are implying that you are being very inactive and probably lazy.
The best thing to do, upon hearing this, is to politely laugh, let them have their fun, and make a mental note to try and make yourself look a little busier.
Though I guess they could be implying you are, in fact, a ship. But that seems like a really odd insult.
Still, you can never tell with these people, so the best thing to do is to laugh and make yourself look busier, and then jump in the water, sink to the bottom and drown, thereby proving you are not a ship.
Then, at your funeral, they’ll really feel stupid.