Betta fish has white growth on his back. What is it and how can I treat it?

My Betta fish has had a white growth on his back for over a week. Pictures linked below. I’m keeping him in a 5 gallon tank and am replacing ¹/₃ of the water bi-weekly. No filter because the one included with the tank ran too hard and disturbed the water and the fish.

I’m worried in general for his health since even before developing this growth he’s never been quick to eat (I give him TetraO pellets and bloodworms). The fish was kept in extremely poor conditions for most of his life and even though they’re better now, I worry it may be a little too late.