Oooo poor pleco. I don't think Melafix is going to solve that, it looks like some serious tissue damage and possibly a tumor.
What has that plec been sucking on? If he was sucking on the heater, he could've burnt himself and the wound gotten infected. If he was sucking on the filter intakes, he could've gotten some of his lip caught in the strainer basket, wounded himself, and it got infected. If there are rough edged decorations in the tank, he could've cut himself on them.
Remove the poor pleco to a quaratine tank, perfer at least 10 gallon. Give him a half-cut flower pot as the only decoration (or any other smooth surfaced rounded edged decor he could use as a little retreat cave) and set the tank up very simply with basic filter (whisper 10s are pretty cheap) and heater to stablize temp. Then I would try to use Maracide, it's pretty strong stuff and good against bacterial infections. You could also try the Maracyn I and Maracyn II tablets. Ten gallons is a whole lot easier, and cheaper, to treat than 55. Feed your pleco softened algae wafers, and some squash or cucumber rounds.
Good luck.
~~Colesea