This is a keratinous horn type grown and it needs removed. They are made of a material similar to your nails. It will keep growing and could catch and rip off. Most are benign but they do need removed with a wide base excision to stop them coming back. How old is your hammy? the vet should be able to give you a prognosis on removal then you can decide the way forward. If they feel it is not in his best interests to remove then at least you will know. Good luck at the vets
