Yes, fungal. There are a number of 'flat fungi' that you find on the underside of fallen wood and that have those tendrils. This is a species Phanerochaete or something that would once have been placed in Phanerochaete, that genus having been split into several. I have a number of herbarium collections of this sort of thing that I'd filed away as Phanerochate but I know that I have to check them to see just what they are.
I'd guess that the wood had been turned over at some stage. The area on the underside of wood lying on the ground is a sheltered micro-habitat, very conducive for such flat fungi - whereas the upper surface is a harsher micro-habitat, exposed to greater temperature variation and drying out.