You would need to make the physical port on the Aruba Controller that the Generic L2 switch is plugged into untrusted.
Before you do that make sure:
- The physical port you make untrusted is a different port than your management port for the controller. You can have two ports on the controller in the same VLAN, only make the one that is connected to the generic l2 switch untrusted. If you make the port that you are managing the controller through untrusted, you could lose management access to the controller.