Looks like water droplets? I know glitter sometimes contains metals, and I have heard of symbols "rusting" (Pinwheel is prone to this!)
It might be a chemical reaction to the plasticizer leaking out and mixing with the glitter. Or just an aging process with the plastic and glitter.
I've seen a lot of ponies with this. Sunlight, Starshine and all of the 2nd set seem to be prone to this!