The crux of this issue is that you are unhappy with it.
Your husband - and indeed his ex - can protest until Doomsday that their relatrionship is innocent, but if it is making you unhappy, then that needs to be addressed.
You should talk to your husband and explain how you feel. He should be sensitive enough to realise that this is making you unhappy, and make appropriate adjustments.
He could - and may well - argue that you should be secure in his love for you, and that his relationship with his ex is exactly that - a relationship with someone he is no longer in lovve with - but the fact remains, it is making you unhappy, and your feelings count.
Talk it over with him, make sure you know exactly what result you want to get from the discussion, and work towards it, but be prepared to see his point of view. Be aware of not backing him into a corner with a 'her or me' choice at the end of it - this situation is not that simple.
If you don't feel able to approach this rationaly - and who could blame you - then maybe you should ask him to go to Relate with you, for some structured and impartial help to get you over this situation.
Let us know how you get on.