Sorry it wasn't showing up when I posted. I can see it now though.
Slime mold? Maybe someone with more experience can say.
I don't think he's getting enough raw nutrition with what you're feeding. What pellets have you tried? Have you tried New Life Spectrum Betta or Omega One? Sometimes bettas just need a few fast days before introducing pellet food. You can also add vitachem to the water. It's a fish multivitamin and would make up for the vitamins he's missing from the food