That is actually the stopper for that whole assembly. There should never be a hole in it. It was put there to stop that pipe from jamming into the water pump. It's not leaking out of there. It has to be the seal that goes between that pipe and the water pump assembly. I used some grey Loctite and the o=ring to help it not leak. The seal kit should be like $5 USD so ya I'd just pop out that pipe and redo the seal.