Damn, this is disturbingly close to "Eats, shoots and leaves"

The comments made me realise there's at least 3 meanings:

Mushrooms that eat plastic. Plastic that eats mushrooms. 'Eating mushrooms' (ie edible) that are made of plastic.

Any more?

Eating mushrooms that are made of plastic? (Likely something from a horror movie.)

Ah something like.

Plastic: Eating Mushrooms (a memoir)

It’s funny because hyphens (hyphae) are also a type of mushroom.

Hyphae aren't a type of mushroom, they're the filaments that make up fungi. A mushroom is a part of a fungi, made of several Hyphae joined together.

Hah! Amazing!

Ok, we've belatedly hyphenated. Good point—thanks!

Let's give that sentence to AI and see what image it produces

Here we go: https://imgur.com/a/v3VhRR9

Created by DALL-E using the prompt "Plastic Eating Mushrooms".

Funny that AI is certain the mushrooms are made of plastic. I don't think many people get this as a first interpretation. (Personally, I got the plastic doing the eating one.)

I like that one of them is a fly agaric (the classic red with white spots toadstool) that's poisonous.

While you can eat them after boiling, it's not really a go-to example for an "eating mushroom". Really shows that there's still a lot of context blindness in AI.

Also it seems to have glommed firmly onto the "mushrooms for eating, that are made of plastic" parse.