British distribution: Widespread
Mucilago crustacea is a frequent slime-mould (myxomycete), appearing on damp grass, including pastures, lawns and amenity grassland. It is commonly said to resemble dogs' vomit and sometimes causes some alarm to owners and users of the grassland, but is harmless and soon disappears.
The mobile, multinucleate stage, the plasmodium, is pale, creamy yellow at the onset of the fruiting stage. It emerges from the soil onto grass and coalesces into a single mass, becoming an aethalium, which may be several centimetres in length, turns white (rarely yellow) and is a mass of tubules. A crystalline outer cortex forms, and very soon this begins to flake away, revealing the black spore mass.
Ing (1999) and Poulain et al. (2011) provide keys to myxomycetes, the latter in both French and English, and the latter publication also includes an extensive atlas of excellent colour photographs.