I am having a coolant leak behind the motor

I am having a coolant leak behind the motor. I had the intake manifold replaced thinking that was the problem but it was not. I have tracked the leak which is coming from a brass or metal pipe that goes into the heater core. A hose goes onto the pipe from the intake, I believe. I believe the leak is due to water being used in the car instead of antifreeze/coolant. The pipe is an odd shape which appears to wrap around the heater core.

How can this leak be fixed or how can I replace the pipe?