First off, welcome to Tropical Fish Keeping forum.
I moved your post into the Tropical Fish Disease section which is more appropriate and where more members may be able to offer suggestions.
A photo would help. Some of our members have considerable experience with fish ailments, more than I have. While treating the pleco, you should also be exploring the cause. Posting the tank info (fish, plants if any, temperature, hardness and pH, ammonia, nitrite, nitrate, water change schedule) will help sort this out.