An unidentified slime mould that is just starting to form sporangia.
Photo taken in Whiteknights Park, Reading University grounds, Reading, UK, on 2011-12-21.
A photo of the same specimen taken on the 2011-12-24 shows the white lumps to have disappeared leaving just a smooth translucent white plasmodium. Either it wasn't forming sporangia after all or else something happened to cause it to give up on that plan.
This might be the plasmodial stage of Ceratiomyxa fruticulosa.