Help removing electrical from old Alternator

Any ideas on how to remove that electrical connection? Seems like it should pop off but it won't budge and I obviously don't want to damage it.

 

That grey thing should pop off, and then there is a nut underneath that secures the cable itself

 

That was it. Just paranoid because that plastic thing wouldn't pop off.

 

Especially if the plastic is old, had to replace mine. :-\