I have never seen a specific tool to move that. I would look around a pick up a knurled one if it is a big deal to you. I guarantee you, that a tool that size would have been quickly lost. If that is factory, not one of Colt's better ideas. What model gun is that, it does not look like a factory rear sight, it looks like some of the after market stuff I have seen.
I have seen that sight before, but never on a Colt.
Does not look like it would be hard to move with a FMJ bullet, like we used to use on some of the older guns.