There is a sac type thing behind and or near my Betta's gill could he be sick?

I just got a Crown tail Betta about a week ago, I had him in my 2 1/2 gallon tank temporary for a week to make sure he was good and healthy before putting him in my 10 gallon, He is very active and lively also very shiny, he doesn't seem sick and I haven't noticed any protruding scales. I have a tank with a divider and a Betta on the other side. When Sydney (my crown tail) saw my other Betta (Morris) he naturally flared at him and I noticed a sac looking thing by his gill, It was the same color as his body (white pinkish color) and I didn't see it after he stopped flaring. Could he be sick or is it natural or just a deformity. Any answer would really help I really care for my Betta's, If he is sick I would like to treat him as soon as I can. Thank you.