it may be a springlok coupling but with the red protector in place.

see if the red bit prises out and then you may see a spring coiled around in a full circle holding the ally pipe in the plastic bit. if so, you need to speard the spring to release it. there are special tools to do it, but you can do it by cutting a length of ally sheet out and wrapping it around the pipe to the right diameter and then push it into the spring. when the spring is released, you can then pull the 2 parts apart

then again, it may not be like that all