Betta Fish: What is on this betta?

One of our store bettas developed this mass overnight a few weeks ago. Despite standard treatment, it has only gotten bigger (treatment as per company protocol is salt and Artemiss).

At first I thought it was some sort of fungal growth. There is a milky-colored fuzz (or maybe mucus layer?) encasing the pink part in the middle. Last week these brown strings appeared, but I have a feeling those may be poop. However, the growth now has two distinct lumps with a bundle of something the same color in the crux of where they meet. The whole thing jiggles when his fin touches it.

Is this just a bad tumor? Is it a parasite or infection? A wound that healed terribly? Other than the lump, he seems very healthy. I’ve just never seen a tumor like this, especially with the brown part and the fuzzy film on the outside.