could it just be silvery coloration to the scales in that area? some bettas get silvery around the gillplates/under their chin as they age. if you notice ANY fuzz/fluffiness to the area treat right away for columnaris (it often starts on the head). keep water quality tip-top (which appears to be at present) and monitor his behavior. any change in feeding/activity level and begin treating right away. not saying it IS columnaris (doesnt appear so from photos) but be on the lookout and be prepared).
he sure is handsome!
cheers
he sure is handsome!
cheers