It looks more like flash then a crack. Flash is raised metal whereas a crack would not be. It's a normal result of the manufacturing process, resulting from a gap in the edge of the production tooling which allows excess metal to flow out a bit. Usually it's broken, polished or machined off later, but not always, especially in non-critical places or where they don't expect it to be visible in the finished assembly, like the back of a crank arm. Don't sweat it, enjoy your new cranks.
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