Its known for leathers to shed their mucusy skin every once in a while, and for them to look horrible while doing so. Ive never see a leather look like the picture you posted if it were healthy. Can you please post what your ammonia levels are reading? If theres any, I would have to say that the leather might have died and now is decaying, hence why you would be seeing chunks of gunk rather than what looks like a large mucusy string attached.