Betta Fish: Infection or coloration?

This is Midas. He is in a 5.6 gallon tank, with a heater and filter. It’s planted with anubias, val, and java ferns. He shares the tank with a small nerite.

Ammonia: 0
Nitrite: 0
Nitrate: 5
Tank Temp: 77.4

20% water changes weekly. Water treated with Prime.

Feed a variety of pelleted foods and frozen blood worms and mysis shrimp as treats. He is skinny, was emaciated when I got him. We are working on gaining weight.

I’ve had him for about two months. A few weeks ago his gills started darkening. Today I noticed brown stuff on his face and body.

I am wondering if it’s just a change in coloration or if it’s an infection. If it’s an infection how should I treat it?