Okay I'm glad to hear it isn't diarrhea. I think it is that scent gland.

Males have the scent glad right at the rear of their rump. They can release a musky smelling scent when trying to mate or some claiming a spot ask their own. I don't know too much about it but I don't think there is too much to know about it either.

To clean it you can give you pig a rump dunk. Put some warm water in your sink - about 2-3 inches of water. Try to see if the oil will come off with just the water.

If it requires more then just water get a mild shampoo, preferably Bunny Bath. If you don't have any Bunny Bath try to use a baby shampoo. You aren't suppose to use human shampoo on cavies because of the pH difference. But if water doesn't work and you don't have any of the above listed shampoo's it would be safe to wash just the greasy area with a dime amount of shampoo.

It is VERY IMPORTANT that your cavy is completely dry before returning him to the cage. You want to make sure he is dry so if he happens to catch a drift he doesn't become ill.