I know these and cardinals can also vary from a deep dark blue to almost green depending on several factors (ex. water conditions, mating, dominant/submissive, lighting). The fact that this one was not with the school may mean that he was a recent addition or stressed out?...just another possibility.
The green neon tetra's blue stripe is straight across the back like a cardinal, while the typical neon has an arch in the blue. Look for that next time you are there.
Green neon:
Standard neon:
Cardinal tetra:
BTW...a bit off topic but I would love to get my hands on the royal tetra. Has anyone seen a fish store carry these? Also know as the Blue emperor tetra:
