Thats not Vishnu. Just Fred. Ya, you know, Fred from corporate? Yeah, he fell into a pool of toxic waste and spontaneously grew 10 more arms. Ever since then he just hasnt been the same. He has terrible back problems.....psst Though I hear he's a killer with the ladies nudge/wink xD
Schmuck in English (I would guess via Yiddish somehow but I don't really know.) often refers to a not so great guy. I'm not sure why it has this meaning versus the German one. (Though I can think of some possibly similar English word connections with slang for certain body parts. Not sure how appropriate a more detailed discussion of that is for Duolingo.)
Schmuck came to American English from Yiddish for penis, where it translates as foolish or general jerk. It isn't clear if it derived from German as "family jewels" or other sources: https://en.m.wikipedia.org/wiki/Schmuck_(pejorative)
In Yiddish speaking circles is still fairly vulgar due to the older meaning of penis, but in broader English, thanks to Mel Brooks et al. it is seen as goofy and inoffensive.
You can't tell, because nouns (usually?) don't look different in the genitive plural compared to the nominative or accusative plural.
But I'd say it's accusative -- if you add an adjective, you'd have ein Dutzend lange Arme (acc.) rather than ein Dutzend langer Arme (gen.)
A bit like in English, where some nouns are simply followed by another noun without "of", e.g. "a dozen arms, a hundred arms, a thousand arms, a million arms".
As opposed to, say, "a lot of arms, a bucketful of arms, a surfeit of arms" with "of" -- those nouns are less like numbers.
Similarly here: ein Dutzend lange Arme, eine Million lange Arme with nominative or accusative, rather than ein Dutzend langer Arme, eine Million langer Arme with genitive.