Its green, somewhat slimy, translucent and apparently edible, and it grows on my flat roof in warm and wet weather. It gets washed off and even blows off, and sometimes the gulls pick it up and drop it – but for a couple of years I had no idea what it was. Then I sent a photo to firstname.lastname@example.org (Sussex Wildlife Trust) and they identified it for me.
Nostoc commune is what they used to call a blue-green algae, but the preferred name is now a cyanobacteria, and according to my Google searches it not only photosynthesises but is also capable of fixing atmospheric nitrogen. It grows all over the planet, and is eaten by Taiwanese (they call it yu-lai-gu or post-rain mushroom) and the Chinese, who call it Tian-Xian-Cai, which translates as the Vegetable of the Heavenly Immortals.
This last name is connected to the idea that it just appears as if by magic, or from the heavens. The oldest English name is star-slime, but other names are jelly star and witch’s butter. In a book of 1640 it was described as ‘possibly the excrement blown from the nostrils of some rheumatic planet’. Nostoc salad anyone?