Oooh. I'm kind of surprised that doesn't ruin your cycle (are ponds "cycled"?).
That does sound odd. It doesn't sound like a tumor to me, almost more like an abscess? Did it have pus, blood, etc in it?
That is really strange. I'm not even sure what that could have been. :sad:
But, hopefully, giving your pond some time with just the turtle will kill the cause off.