Beautiful
betta boy!!!
I don't know of any parasite that would do that...maybe a bit of blood, or a color change is what I would think. Are there any other tank mates in the aquarium? Something that might be nipping his fins?
I would do some small
water changes every other day to keep the water pristine. Clean water helps to prevent a multitude of problems. If you don't already have some, I would also recommend adding some Indian
almond leaves to the tank. The leaves will release tannins, which have
antibacterial and antifungal properties. And bettas just love the stuff. Tannins help with fish stress, too.
If it spreads or gets worse, and becomes apparent that it is not a color change, then I would dose with some
aquarium salt (as long as you don't have any invertebrates in the tank, or any other fish that don't tolerate salt).
Also, check out any decor that might have rough surfaces in case your betta got hung up on something.
Hope this helps!