http://en.wikipedia.org/wiki/Lymphocystis
http://www.tropicalresources.net/php...er_disease.php
There's some more info.
I'm not completely convinced that's what it is, mostly because of the type of fish, but I don't have any better suggestions.
It could, like you originally suggested, be some kind of tumor. It could have also been caused by some kind of irritant.
Either way I can't suggest any type of treatment because I'm not completely sure what it is.