This was an interesting spotting, can't make anything of it... It looks like a 1 to 2 cm thick white and compact talus, hard on touch. It spread on a fallen tree and covers some 75 to 80 cm of length, with some 10 to 20 in large. On edges there are some carpophore looking growths (Ph N°4).
Growing on a fallen tree in a wooded zone; the area of mixed (but in majority broad leaved forest) woods on banks of river Rhône, in a protected natural reserve.