What is this slimy coloured stuff growing in my plates? Recently, I observed that there is some brown slimy stuff on the underside of my plates. I don't know what they are or how they form. I tried removing it with a tooth pick, but was too hard to come off. Does anyone know what this is? How can it be cleaned?
My first guess is oil deposits that have hardened through oxidation. Especially the highly unsaturated oils are prone to oxidation, which will after a while lead to insoluble deposits (like in the original oil paints, where the linseed oil polymerises after prolonged contact with air). It's hard to remove as it's not brittle, and deforms rather than breaking, like an oil varnish.

(are those items you show plates, or pan covers/lids?)

It doesn't look like any kind of mold.