strange sac under chin


hey there...

my severum has developed this strange sac-like thing under his chin...

he always had a bit of a scar there, but it never got any worse or
better...

then just this week it's like it blew up and bubbled out so now it's
kind of like a smaller version of those weird bubble things on
goldfish...

any ideas what it is? should i be worried? how can i get rid of it?

thanks!
.