Mouse nest stuff should never make it past the piston rings and into the engine oil, or at least I'm not seeing how. The only path I can see is either through the oil fill hole or if say one had done a valve check and left some paper towel or shop towel in somewhere...
Next step, pull the valve cover..... keep reading.
If one could identify the type of fibre, might help, but it's soaked in oil... perhaps take a sample and rinse it with naptha, then isopropanol to take the oil away. Lab analysis best, but I'd like to know if it is plastic.
I don't want to unnecessarily alarm, but the stuff had to be picked up by the oil pickup screen in the pan and pumped to the filter. There is a source of plastic that could be ground up fine like that..... cam chain guides.
On the underside of the valve cover there is a "guide", right above the cam chain to prevent the chain from hitting the underside of the valve cover. Cam chain is very close to the cover here and I guess Yamaha identified slack would occur between the two camshaft sprockets. This thing is a press fit of (I think) two nubs on the 'plastic' to mating recesses in the valve cover, no adhesive. A not so great snap fit is another way to describe it. Quite rare, but they have fallen off, tight clearance to the chain so it would grind away...... either that or the cam chain is stretched to the point of intermittent contact with it..... shown here as item 6 (Guide, Stopper 2).
I have had a couple fall out while cleaning up the valve cover and they will stay if pressed back in but it's not what I would call totally secure. Any I put back in, I clean the area well with isopropanol and use some RTV. A member in one of the sandboxes, after doing a valve check, could not get his engine to turn over, and this part was jammed down in the crankshaft sprocket. It must have fallen out as he was replacing the valve cover.