It's not mold. it's a result of moisture absorption, but the deformity remains ever after the moisture has evaporated.
We tried soaking some SNES cart shells in a bucket of water in a backer of water a couple of years back in order to make label removal easier (we've since found a way better method). About half of them had the blisters the next morning. Even after we thoroughly dried them, the deformity remained.