Is there a term for your parent's ex-spouse who is not your biological parent?
Solution 1:
It's completely appropriate, if you desire, to continue to call him your stepfather if that's what he was. You can have several step-fathers in succession. Simply because you are no longer living with him doesn't erase your relationship with him. Your mother doesn't become your ex-mother when you leave home. Step-siblings are also for life.
If you are not close to him, you can call him your mother's ex.
If Heather can have two mommies, you can certainly have two step-fathers.