I have delusions of being a competent recogniser of things that turn up in UK gardens, but when this started growing in what had previously been a pot of basil in the garden, I was clueless:It looked a little like moss from a distance, but the bibbly bobblies were most certainly not moss-like close-up, and the whole thing had a rather unpleasant squeaky texture. Touching it left brownish-green stains on my fingers. Definitely not a moss.
A clue to its identity came when I unwisely tried poking about looking for roots, and realised it was growing on a cat turd.
Having suppressed my nausea and thoroughly washed my hands, a quiet bell from the mycology lectures I took during the Bronze Age rang. A vague memory of pin-moulds, big as your head, grasping for the light.
And indeed that is what this was. Phycomyces blakesleeanus is a giant relative of the tiny pin-moulds that grow on damp bread. A welcome addition to an otherwise lifeless and drab winter garden.