Why does Ganta have hexagons around his blast?
This isn't really explained in the show. As you can see, in the image below, the Branch of Sins manifestation is based on the power of the user.



However, if we take Episode 1 into consideration, we can see that Wretched Egg attacks Ganta and kills off his class mates. This is also what seems to be when Ganta should be receiving his powers. In that case, since Wretched Egg's powers have the hexagonal patterns around his/her abilities, then this would explain why Ganta has the same design, since Ganta got his abilities from Wretched Egg.