Q What is this green slime growing on damp pebbles?
Susan Pearce, by email
APIPPA SAYS This is a cyanobacterium or blue-green algae known as nostoc – although more imaginative names include star jelly and witch’s butter. It appears in my garden too, usually on compacted surfaces like the driveway or paths. In wet conditions it becomes inflated and slippery, then in summer it dehydrates and disappears… often reappearing in autumn. If it’s in an area where it could be hazardous, you could scrape it up, but you may have to do this repeatedly. I don’t bother – instead I just walk around any patches when they’re slippery!