You can see fungi growing out of dead wood in just about any woodland setting but to see one growing out of the side of what appears to be a living tree is most unusual. The elm leech (Hypsizgus ulmaris) is an uncommon fungus that was widely reported during the Dutch Elm Disease epidemic in the 1970's on dying elm trees but it can now occasionally be seen on oak, poplar and horse chestnut. It is not a species that kills trees but one that soon gets established in a dying tree.
It grows in small tufts but quite often in several groups on the same tree and appears in autumn. It can be eaten; the flesh is white but tough so probably not worth it.