A sealed LP is not necessarily factory-sealed. Every Tower Records had a resealing machine in it’s backroom, used to make returned LP’s sellable again.
In the 1970’s there was a company (Recoton, iirc) making plastic-lined sleeves. The problem was, the plastic was of a type that "leeched" onto the vinyl, leaving a dull residue. That’s exactly what your LP looks like.
We quickly learned to not use those sleeves. The "rice" paper sleeves being made now (MoFi, Vinyl Storage Solutions, etc.) do not have that problem