A white jelly fungus, usually more or less opaque and fairly firm in texture.  It may have yellowish or grey-blue tinges.  The fruiting bodies are cushion shaped, becoming convoluted and merging with adjacent ones to for a larger mass. 

Crystal Brain (Exidia nucleata) has opaque white inclusions and is usually less firm and more translucent

Photogrpah in habitat; note substrate and associated tree species. 

Mainly on dead and decaying hardwood, particularly Beech and Ash.

Late autumn and early winter in Britain.