Angel, a few of mine have discolored in such a way too, even after completely cleaning it off the color is still "off".

I had a girl at my local sex shop tell me that I should never keep any of my "jelly" toys laying next to each other uncovered, as the different kinds of material can "melt". I have no idea if this is true, but I do have a a few butterflies that have small black stripes on them from exactly where the cord was laying against them (and i've not gotten the chance to even use the one yet)