In color prints, just about everything about the final print is
influenced by settings used on the print-making equipment. In that step,
the photofinisher can change the contrast, the color balance, etc. In
most cases, this is all handled automatically, with little or no human
supervision. (The better photofinishers, like pro labs, will have a
trained human watching the process and correcting the automatic settings
as needed.) Thus, if you're seeing a green cast, one possibility is that
the machines simply got confused by an unusual set of colors in the
negatives and caused the problem. If so, taking them back and getting
reprints should fix the problem, assuming they actually monitor the
reprint process.

Another possibility occurs to me, too: The film might be old (that is,
beyond it's "use by" date when exposed and/or processed). As film ages,
the sensitivity of the different colors changes in different ways, so you
can get odd color shifts. I'm not sure exactly what's common as color
print film ages, though, so I don't know if a green cast specifically is
a likely result. (I got a blue cast from an old roll I recently tried.)
The photofinisher might be able to correct this problem in printing, so
it might be worth taking the negatives back and asking about it.