Why is this emitter leaking?

This is not water intake this is water output. I've noticed continuous leaking here and nowhere else along the path. It hasn't even been turned on for weeks given it's been raining. The photo will show a lot of water but that's because I've been fiddling with it. The second photo just shows where the intake is coming from to prove it is downstream of the regulator valves.

How could there possibly be a leak? It must be leftover water from further downstream right? So if I were to loosen the cap it should eventually stop? It's scary doing that because it seems to be a lot of water and it's hard turning the water main off with a whole family at home.

Sorry if my terminology is wrong I don't know the right words for these!