Look at the skin underneath the crust. If it is healthy and pink looking, it is just something some puppies may get similar to cradle cap. You want to use something very mild to clean it up an make sure it is well rinsed so mum doesn't get bothered by it.
You can also use some mineral oil or olive oil to loosen it up and pick or comb the crust off. Usually a one off thing, not contagious and the puppy won't have it again