Trichaptum biforme (also known as Hirschioporus biformis)
Widespread and very common this thin, narrowly-zonate, bracket fungus fruits on deciduous logs. The upper surface is more or less flat and velvety. More often than not algae grow over the surface of the bracket and it turns bright green. The giveaway is that underneath, and sometimes above, there are distinct violet to purple shades especially near the edge of the bracket. If it is similar to this species, but has white to cream pores underneath, then it is probably Cerrena unicolor. Note the purple edge to some of the brackets in this photo.