Huh, did he peel off his own nails? Why?
It's the evil slough thing, where you discard stuff like hair, nails, teeth to sever the connection between you and something impure.
A chapter or two ago we saw a flashback where his grandma was cutting his nails pretty severely while a fucked up thing was lurking over their heads.
My interpretation was that he's stuck in a cycle of:
1. Fucked up thing starts following him around.
2. He cuts his nails and maybe does something with them causing the fucked up thing to go after the nails instead of him.
3. Either that thing stops following the nails and comes back OR a new fucked up thing notices him.
Furthermore I'd guess that the nails being bloody is sorta mandatory? Like there's probably a reason he doesn't just use hair.