Pony Cancer is a derogation of the plastic, so it won't spread to other ponies. If they are stored in similar conditions, some of them are more prone to it, but it can't get from one pony to another.
I hate to say it, but it does look like age spots of the plastic, at least to me
![Sad :(](https://mlparena.com/Smileys/default/sad_001.gif)
It is possible that they won't get worse than that. I might be wrong though as the plastic used for Greeks is a tad different than the regular one :/ Have you tried cleaning it anyway?