That looks like his scent glands
He should have one on each side. It's quite normal. Sometimes they lick them and sometimes they look a bit wet or greasy. When you see them rub up against something in the cage, they are scent marking it - scent marking helps them find their way around the cage and know where everything is as their eyesight is so bad. It's why it's not usually recommended to do big cage cleans or cage cleans too often, because it removes all the hamster's scent in one go and stresses them because they can't find anything!
Doing partial cleans is a good idea - ie doing the toys or wheel on a different week to the substrate. I mainly spot clean the cage and only do a big clean every 2 or 3 months usually, and even then I only replace the top half of the substrate (helps if you have deep substrate then the bottom half stays clean and still smells familiar and the hamster can dig and burrow a bit).