It is pica, as pica is just a term for "eats stuff that isn't food." Pica can mean eating all types of inedible things, but it can also be eating just one particular inedible thing. In fact, in humans, there are "subtypes" of pica based on what type of thing the person feels compelled to eat, see https://en.m.wikipedia.org/wiki/Pica_(disorder)
I would take your cat to the vet to double check it doesn't have some sort of underlying issue that might lead it to attempt to eat plastic, which is sometimes the case with pica. You should especially take it to the vet if it is a recent development.
However, sometimes pica really is just an inexplicable compulsion. The only treatment then is to make sure you keep the object the cat wants to eat, in your case plastic, away from the cat.
Plastic, especially plastic wrap from packaging, or plastic bags, are actually pretty common targets for cats with pica. Many cats are attracted to the crinkling noise these plastics make, try to play with the plastic, and then end up eating it. Therefore, it may also help to find other things to entertain your cat with, to distract it from attempting to eat plastic. You could even give your cat toys that are made to make that same crinkly noise.