We have no idea. If it's important it will most likely come up in the Greed novel. Though we have seen that Rahab Barisol's genes seemed to be active in Kagura and Kayo's bodies even though Kagura had black hair so maybe the green hair comes from the Eve part of Ma. We have no canon answer though.
Considering how often genetics are a plot point (ex, Prim had to hide Ney's existence from her husband because the blonde hair gave away it was Arth's kid), I'd say it probably doesn't.
I'm not sure if this'll end up being the answer but Michelle's hair shade, and Nemesis' hair shade, are both considered "Elphe Green", and genetically similar enough that people like, say, Hanne and Heidemarie can pass as sisters. So it's possible that Nemesis inherited the hair color from the same part of the family that Michelle did (assuming she didn't just get it from her mother).
Yeah, given the presence of two characters in the same bloodline who share a single parent and both have a hair color that's similar, it's far more likely that Gallerian has an Elphe somewhere close in his bloodline than any weird magic going on.