Your hamster has a gene called white belly (Wh). It is perfectly harmless!
However, if you wanted to breed, knowing this would be VERY important. If you have another ham who was identical to your ham, but a girl, they would have a 25% chance of having babies with DOUBLE white belly (WhWh) and those babies are born completely white, with no eyes, usually deaf, and often times no teeth.
It's not a harmful gene on its own, and can make some hamsters look very pretty! But it's very important for a breeder to know about, and a reason why breeding should always be left to those who are very knowledgeable in genetics.
Your fella is a handsome chap. x