And that mark is still on him and the reason why he can be tracked by the walkers. Benjen said the wall is protected by magic and he can't cross it. If Bran cross the wall, it may cause that protection to disappear in the same way as the cave of the Three Eyed Raven.
That, or someone has their hands on the Horn of Winter and it will be moot. I suspect one way or the other we will have a "collapse of the wall" scene before the end of this series.