Fungus Kretzschmaria deusta (= Ustulina deusta) growing on a decaying log. This fungus can dangerously weaken tree trunks, causing them to break. The white and grey growths are the immature fruiting bodies which appear in the spring. As they mature, they harden and turn black, as shown in the following photo:
Photos taken in the Wilderness, Whiteknights Park, Reading University grounds, Reading, UK, on 2010-03-23 and 2010-03-28, respectively.