I believe it's green or turquoise. I'm sure the severum is the aggressor. It chases the poor green terror around the tank. It approaches it then pushes it out from where ever it is.
It looks to be a Green to me as well. Just in it's stress markings in the picture. Cichlids are Cichlids, they all have personalities. Also when mixing larger cichlids even Severums it is best to get them young and have them grow up with one another (before they hit the 2.5-3inch marks on something like a severum).
The Sev might have claimed the entire tank to itself. If you decide to replace it I would replace it with some fast moving dither fish for the top layer of the tank. It will help the GT not be as shy as well as provide a more balanced look to the tank.