It sometimes happens to clams. If it is healthy and all it should only last for a couple of days max. It has happened to me, and I am unsure of what or why it does that... could be anything from irritation inside the clam to a parasite. If this clam of yours doesn't look any better in a couple more days post a pic so we can get a closer and better look at it. also as mbbuna says, it would help a lot to post your params and neighbors of this clam.