How would one go about complaining to the boss about this?
"You increased my salary to 50k and hired a new guy who you are also paying 50k. Even though you just gave me a small pay increase from the salary we had previously agreed upon, now that the new guy is making 50k, I need to be making more."
If I'm the boss I'm gonna tell you the new guy's salary isn't your concern and you're asking for a raise but haven't given me any meaningful reason to grant it.
You could go tell the boss that you feel your work has been more than satisfactory for a long time and you believe a salary increase would be appropriate. Because4 now you're actually explaining why YOU should get a raise, rather than talking about the new guy's salary.