What I think is one of the stages in the lifecycle of slime mould Enteridium lycoperdon (= Reticularia lycoperdon).
When it first appears, this species looks like a small pile of grated coconut. It then consolidates into a solid globule and develops a silvery crust which then cracks open to reveal a fine brown, powdery, mass of spores.
The following specimen appears to be slightly less mature than the above one.
Photos taken in the Wilderness, Whiteknights Park, Reading University grounds, Reading, UK, on 2010-04-11.