Ringworm isn't as contagious as many people believe. The best thing you can do is take the rat to a vet, they will scrape the affected area and be able to tell you if it is some form of fungus. It may or may not be ringworm, but it's an idea. I think if it doesn't clear up in a week or two, a visit to the vet is in order. Also, normal quarantine practice should apply in case it is something a little more serious.