That old classic mold again! Mold seems to love disks, once it gets in, it will spread and munch away if the conditions are right. I had this problem some years back. I took an a1200 and imaged thousands of disks, this was back in the days before Toni Wilen took over winuae. I then stored these disks away in old boxes. Conditions were not ideal and I lost around say 50% of them to damp and mold. But because they were imaged at least the data was not lost
When I acquire bundles of disks from ebay or other sources, some are perfect and others are not. I have 5 here that are BAD. I would not even put them near other disks until I had cleaned each one inside and out.
All part of the fun I guess!