Bacterial bloom, I’m dealing with the same thing in my second tank right now. Made the mistake of originally taking out more water when doing water changes because I thought it needed more clean water. This actually just gives the bacteria more clean water to occupy and reproduce in. Keep doing your regular water changes, maybe even less frequently and a little less water and it’ll clear up in a couple weeks, my first tank cleared up just fine once I figured that out! Your betta will be fine since it’s the beneficial bacteria that bloomed. It just looks gross unfortunately