Mycena spp. 'bonnets'
Mushrooms are actually only small parts of what can be a very large organism. A field of mushrooms is often really just one huge mass of fungi connected like veins underground, using acid to melt down and soak up any organic material in the soil.
Varieties of this fungi have been called ‘death cap’ and people have had to have liver transplants because they mistook them for an edible variety or thought they would give them a psychedelic trip if they ate them.
To stop things eating them they have toxic chemicals which interestingly enough are the same sorts of chemicals in some soft corals (sesquiterpenoids).
These fungi also have relatives that live out at sea, which means although small. there are mushrooms on the reef.