Sorry to hear. I have no experience with melafix. What is exactly wrong with it? The only advice I would feel comfortable giving is to keep your water param. as perfect as possible. Maybe even daily water changes. Sorry I could'nt be more help.
My plecos have had quite large rips from various issues in the past and have always healed back 100% from using nothing more than stress coat and water changes.
bacterial infection more then likely. w/out pictures hard to identify. I've only delt with Vamps at work and never personally owned one. Melafix is generally a good curative, But will cause your O2 levels to drop. Oxygen levels are why one person told you "hardy fish only" increased flow via a powerhead or bubble wand can minimize this potential problem with the product. and more o2 to sick fish is never a bad idea imo anyways. what are your water peramiters atm? high ammonia or nitrites can cause a poisoning in sensitive plecos. I'de also bump the disease board for a quicker response. like I said I've only delt with them on the job, and i'm pulling from experiance with sensetive species. Hope someone w/ more personal experiance can bump this. But there is a start. I also dose 1/2 reccomended dosage first day of treatment on sensitive species and play it by ear w/ Melafix. But it is personally my "drug" of choice. Though i woudn't reccomend treatment until you've done a diagnostics of your system's water. and ruled out water quality, temp ect.
You'll want to find out what exactly that white patch is before adding meds though...last thing you want to do is stress your sick fish by treating it with the wrong meds.
Post pics if you can, and make a post in the freshwater disease/illness forum w/ the pics / explanation / and list your water parameters/etc.