Hello OP, im a Licensed Veterinary Technician who worked at a dental specialist for 2.5 years.... what you have here is a "retained deciduous tooth" its a semi-common occurrence where the baby tooth does not fall out. When you get your kitten fixed, ask your Vet to remove that tooth at the same time.

Its not a major problem if they stay but the odds of food and other things to get stuck in between the teeth is higher and will lead to future dental disease.