A Myxomycetes (Slime Mould to general naturalists) "Myxo" for short.
Superficially like fungi, they are far from being related.
This one is Reticularia lycoperdon
I don't know if it was a slug/snail which took the white surface off, or if the dark stripes
appeared by some other means.
By 10.5.14 all the white surface had turned brown