Sorry, don't have a solution for the problem but some on Audio Asylum have reported similar problems when leaving LP's on sorbothane or similar sticky mats like the None-felt.
Problems with Sorba gel mat anyone?
I think I've made a mistake by leaving some of my LP's on my TT for a few days. I have a AQ Sorba-Gel mat. On the records where they have the most contact with the mat, they are developing a white "ish" almost mould like growth (can be seen at an angle in the right light). This won't come off with my usual cleaner. Anyone else see this before? It degrades the sound greatly.