It's probably mold and this is not uncommon for vinyl if not stored properly. Humidity in the cardboard jacket and a nice dark possibly warm from you tube amps--well just a perfect environment to grow mold. Many cleaning fluids have an anti-mold agent in them. I think the deep clean disc doctor does and many goners recommend this. I make my own cleaning fluid (tisk tisk--the shame the shame), and I do use a very small amount of anti-mold agent.
A good professional cleaner like the VPI 16.5 and a fluid with anti-mold agents should take care of it.
A good professional cleaner like the VPI 16.5 and a fluid with anti-mold agents should take care of it.