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Could be the outer layer and old layer was never well separated,thus removing it removed some of the new layer making it look like he has rubbed it off while flicking hairs. I want to add,this doesn't mean in any way that I'm right but it looks similar to normal wear and tear of an older exoskeleton.
If you zoom in,you will see actual stretch and exoskeleton breakage and I suspect when you lifted the old exoskeleton it caused this and the lump which is just compressed soft exoskeleton due to the pulling applied. Its like scraping old paint off,in certain areas it will lump up as you scrape it.
and when i slowly pulled the old exo off it wasnt tht far up on the abdomen it was lower than tht more where it is whiter on his abdomen but i guess its possible there was a piece higher up