Rubber plecos are not very hardy and often times if their needs are not met will waste away. They must be kept in super clean swift moving water. It sounds as though yours already has the ich, which they are very prone to getting. They are such a delicate fish I hardly wish to recommend any sort of treatment. I won't give any advice as to a treatment until you give more specifics about the tank itself. IE, how long has it been set up, what size is it, maintenance, other inhabitants....etc.......