Found a neat scientific paper written about comparative study between Marmokrebs and Procambarus fallax.
In short, the research concludes that Marmorkrebs is actually a form of Procambarus fallax, and NOT alleni or a cross. This form of Procambarus fallax obviously has the ability of parthenogenesis- and although it exists in aquariums and said to be found in the wild of Japan, Germany, Italy, Madagascar, and the Netherlands- a natural population of them are currently unknown.
Here's the paper if anyone wants to delve that far: http://www.ctoz.nl/cgi/t/text/text-idx?c=ctz;sid=85b8cb4e58ea2be46fd44ad7733e3272;rgn=main;idno=m7903a03;view=text
So, although the paper does not mention a correct way of labeling them, I suppose it could be something like: Procambarus fallax var. Marmokrebs