Possible crack in the engine block?

Looks like a typical casting flaw. The block is cast in "sand" that they compress into a mold. Once they press one part into shape they then set in the cores and add more sand and compress it until the mold is filled. During that process a code might scratch the sand or the mold they use to press the sand into shape might have gotten scratched. As long as it is only cosmetic, they generally don't do anything with it. In your case it's simply cosmetic.