tufacody,
+1 That's exactly what I thought. I have removed egg masses from other female fish that did the same thing - broke right through the body wall. It is almost like the egg mass had turned to amber. The only problem is that the masses that I've seen before had all been a bit further back on the fish's body, but not all that much. Everything else is nearly identical, down to the way the lesion first starts up. Perhaps in this clownfish, there was some physical reason why the egg mass was pushed forward? If it was a tumor, you'd expect to see some vascularization involved. It isn't a parasite, but it could *possibly* be a sterile abscess of some sort - did you try cutting the mass open?
Jay
ahh, I do need to add that this has always been fatal to the fish I've seen it in.