Funny growth on pl*co

Hey all.

At work, we have a common pl*co, about 4" long, who lives with a few
Tropheus duboisi (sp?). Actually, it's down to one lonely Tropheus, but
that's another story...

Anyway, on his back, he has a soft, deep red bubble or growth, about 4mm
in diameter. I thought it was a parasite at first, but it appears to be
growing out of him, so it'd have to be an internal one. It doesn't
appear to be bothering him at all, but it bothers me, as (1) I've never
seen anything like this and (2) it has gotten there, or at least gotten
big, very quickly, as I was just futzing around in his tank about 5 days
ago, pulled him out for a few minutes, and admired his coloration (he's
very light and heavily banded for a regular plec) and noticed no funny
bump.

Anyway, if anybody has any ideas of what this may be, or things to look
for, it'd be appreciated. Sorry, no pic at this point, my buddy's
digital camera was lost in an unfortunate canoeing accident :-P and I
was in the habit of mooching his for stuff like this.

TIA,
jesse joe