It looks like a bacterial node or a discus pimple to me Patty . Ive had them in the past , they come and heal away on their own with just clean water , leaving no harm on Discus .
They are usually associated sub par water quallity and lowered immune system just like the Hith and flaggelates you were treating before this are .
Now the part that puzzels me and I unfortinaltely can get answer to is why this is happening in your perfectly clean and maintained tank .
I wonder if anyone else can find the real reason for this problems you've encountered in your very well maintained tank .