A white bracket fungus, probably Trametes gibbosa, growing on a horse chestnut log (Aesculus hippocastanum). The green colour is due to algae growing on its upper surface.
I took a specimen. This is the upper surface:
And this is the lower surface:
There are about 2 of the slit-like pores per mm.
The specimen yielded the following spores (magnification x600, field width 86um):
These average about 3 x 1.5um which is a little smaller than the 4-5 x 2-2.5um given for T. gibbosa in this page at Mycobank.
First photo and specimen taken in Whiteknights Park, Reading, UK, on 2014-01-18.