Nasty fungus

We have some retaining walls, created using railway sleepers.  When pruning a shrub yesterday, which had overgrown the sleepers, and therefore created a moist and shady corner underneath, I discovered a horrible white fungus.  It was in a long layer, about two feet long, fairly flat, like someone had poured some white paint or cream along the ground behind the sleeper, and which had grown in between the cracks in the sleepers too.  It has also eaten into the sleepers.  Any idea what type of fungus this might be, and are there any treatments to prevent or kill it, but not our plants/