I don’t see what you’re talking about. Is it a vertical line down the back from head to tail or is it a ring around the body behind the head? If it’s the first one, then shrimp get a white line occasionally as they grow called a “racing stripe”, its normal and nothing to be alarmed about. If its the ring around the body, it just means it will probably molt soon.
throwaway_brandnewbi 
I don’t see what you’re talking about. Is it a vertical line down the back from head to tail or is it a ring around the body behind the head? If it’s the first one, then shrimp get a white line occasionally as they grow called a “racing stripe”, its normal and nothing to be alarmed about. If its the ring around the body, it just means it will probably molt soon.