I have to agree, small rodent nest. If your clutch pedal feels spongy, or you have some slippage, what ever that material is, there's a good chance some is wound up in the pressure plate, not allowing it to fully release. More than likely, the snap in plug is missing on the side of the bell housing, that is the entry point. If the clutch appears weak, the only way to get that material out of there is to split it, and pull it out.
I had a Case 310B backhoe, with a similar inspection port on the side of the bell housing. Chipmunks had built a nest in there, and it got wound up in the pressure plate. Had to split it, removing the hoe,and loader to be able to do that,and ended up with a 10 qt. water bucket full of string, nut shells, and carpet ravellings out of there. The good news in my case, I didn't have to buy a new clutch & pressure plate, but did take 4 days by myself to tear down clean, and put back together.
This is the oem part # 57579DX I did a quick Google search, and this is the best price I found, without looking too far.
Farmall IHC 1 , 13 , 2 , 23 , A, AV,B, BN,C CLUTCH DUST COVER 57579DX Steiner has the for $12+, others up to $21+.
Get one of these, and make sure to keep the inspection plate you removed in place, and you're good.