A "nutria" is some kind of South American rodent with thick brown fur, once very popular in the fur trade in New York. I've been told they can still be found in the French Quarter of New Orleans.
I seem to recall seeing this is a museum somewhere. Yes, this is, erm... WAS part of New York -- downtown Brooklyn specifically, but Gothic Alley and Nutria Alley haven't existed since early 1940s or so. Urban renewal, new housing projects and the Brooklyn Bridge and BQE remapped huge sections of the city.
I don't know you well enough to guess... what's the appeal of lost alleyways? :D