Could be anything but if the customer bought it as such, then it probably is a taeniatus. Don't know about the green part tho.
It's not happy where it is and if it's a Pelvicachromis taeniatus, it's probably wild caught and wants soft acid water. Unlike the rainbowfish it's in with that prefer neutral to slightly alkaline water.
You need to get it into a well planted tank and let it settle down for a week or two and see what it looks like them. But it's not going to show much in the current tank.
It looks like it could well be taeniatus, and a male. With that quick a clip, the other fish and the state of that guy, there isn't much to go on. The customer may be right.