Well... at the time of the civil war, some cannonballs had a small moldseam, but the great majority didn't. Ordnance Department regulations specified that any "significant" moldseam was supposed to removed at the ironfoundry after the cannonball came out of the casting mold. A "projecting" or ragged-edged moldseam could snag on a powderbag (the cloth bag of propellant gunpowder which fired the ball out of the cannon). Also, a large-enough projecting moldseam could cause the ball to jam in the cannon's barrel during loading.
That having been said, I should mention that a larger moldseam was allowed during the Colonial era than in the 1800s, because in the Colonial era the ball did not fit as tightly into the cannon's barrel as it did in the 1800s.
As Tamrock guessed, I have to say the ball in the photo does not have the look of a cannonball, more because of the ragged hole in it than the presence of a moldseam. But I don't want to jump to a conclusion when the most important evidence is still missing. As the educational article I co-wrote says, super-precise weighing and measuring are needed. Hopefully this ball's finder will read the article (for free) at the link given by Ffuries, and tell us the result.
