There is a condition called canine hypoplasia which causes a lack of enamel on one or more teeth, all in some dogs. There is no cure, and the teeth cannot ever be cleaned. It is considered hereditary although can 'skip' generations, so badly affected dogs should not be bred from.
Unless the tooth is causing discomfort then I would just continue cleaning the teeth as normal and make sure that any plaque is removed, either by cleaning or by giving bones/chews, which will help stop the tooth deteriorating.
I have known dogs badly affected with this, but the teeth are still strong and stay in place all their lives. It is just not very nice to look at.