It's one of the many species of what are called 'slime molds'. Not a mold or a fungus, but a simple organism that 'creeps' along surfaces of all kinds, feeding on organic dusts and particles along the way. Slime 'molds' are usually considered beneficial decomposers.

I've seen them grow on the soil, mulch, leaves, bark, walkways, stone walls and other assorted surfaces. Finding some at work in your garden is normal, and a good thing.