Some piggies can get moles but I'm just thinking that even if it is something harmless like a mole, the fact it's growing and it is near the nostrils would make me want to vet-check it so they can reassure you about how big this might get and whether to be concerned.
The other thing to consider is whether it is actually raised like a pimple or whether it is just an increasingly bald patch that makes it look like it's raised. That could be something different.
They are such cuties! Look at her little friend trying to get her nose in for the lettuce
Have you had them long