Here's another pic to compare it to:
Tumors are strange.... depending on where they're located, they're either horrible, or just... there. My Vulcan's is located in a spot that it doesn't bother him at all, he just keeps on going. He IS losing his dorsal fin, but so far that's the worst that's happened/happening. He's had this since at least.... maybe December? But once again, he got his in a lucky spot. Other people have gotten them on their betta's faces, which can fall off and actually can rip eyes, gills, and fins off of bettas.
I don't think there's anything you can do if your betta has a tumor like this. I've tried. I've tried, and I've tried, and I've tried, and finally, I decided that it just wasn't going to happen. I was stressing my fish out more than I was helping him.