Maybe the sailfin thinks he's a piece of algae and one he gets close realizes that he's not? The goby is green...lol. I don't know if fish can see color. That was more of a joke, but who knows??? Tangs can be bossy. So long as there isn't any stress involved just keep an eye on them. Certainly the goby doesn't seem to be affected by the nusiance of the tang.
When you rearranged everything, did you place anything new like a rock, coral, or plant? It's worth a shot. If that doesn't work then you've tried and they'll just have to deal with each other...lol. Hopefully it never amounts to anything serious.