This is not an animal, mineral or vegetable. It’s not a fungus, either. It’s a Slime Mould. Slime Moulds spend most of their time as single-celled organisms living independently and feeding in the soil on bacteria, fungal spores and other organic material. When they are ready, they gather together and emerge, in this case onto wet grass, and produce spores so they can reproduce.
This one is Mucilago crustacea, the Dog’s Vomit Slime Mould.
Charming, isn’t it?