The white stuff...
8 cm , tipo repisa sobre tronco en descomposición, duro
Updated photos added 13/3/2024 after (during) rain. Surfaces facing downward now appear rippled, margins continue to be irregularly fluted and now showing some rusty red colouration. Original 2 photos at end, taken under dryer conditions.
6/3/2024 Something like this was previously identified to me as Auricularia pusio, but I have never seen these in anything but this dry, papery form.