You are discribing 'popeye'. It is a symptom of many kinds of diseases. So, we need your ammonia, nitrite, nitrate readings. Also, how often to you change your water and how much to you change. What are your molly's tank mates? How long have you had the tank set up and how long have you had her?
No, it is not contagious in and of itself. But, because it is a symptom, the disease could be in your tank. Get back to us asap with all the above info.
Cathy