Thank you Bo for your question to "Ask Extension".  It looks to be a slime mold.  Which specific slime mold it is I cannot tell as I am not a trained mycologist.  A slime mold is motile, but moves quite slowly. It is not harmful to animals or plants and usually vanishes in a short period of time. Slime molds feed on bacteria, fungal spores, and smaller protozoa found on organic matter. Slime molds feed much like an amoeba feeds; they ingest their food and then digest it (unlike fungi, which digest and then ingest). If conditions are favorable, these slime molds will produce reproductive structures (sporangia) that produce spores. When conditions are unfavorable (loss of food, dry conditions), the plasmodium will form hard, dormant, protective structures called sclerotia. Inside the sclerotia the plasmodium will divide into cells containing up to four nuclei. When conditions become favorable each cell will form a new plasmodium. A slime mold is primarily an aesthetic problem. It can be physically removed, but more is likely to return.  Removing all the bamboo litter (the food source), digging out the stumps and drying out the area should be sufficient enough to replant a garden.
     I hope that you find this information useful and as always Happy Gardening.