What is it and how do I clean it.

[ringwraith10]

While it's probably cancer/age spots, I would still take a look inside the pony to make absolutely sure it's not mold. It's hard to tell which it is from a photo, and if it's mold you definitely will want to clean the inside of the pony.

[Elfpony]

Neither cancer nor age spots will spread from one pony to another, so she should be safe with the rest of your herd.

Elf