Fuligo forms a flattish, cushion-like fruitbody- not the case with Arcyria, where the fruitbodies are stalked and from roughly spherical to cylindrical in shape (thus far the latter in the sightings so far on CNM). If you check the Arcyria sightings you'll see that fruitbodies may be closely packed but you are often still able to distinguish the individual fruitbodies.