I freely admit it may have been too long since I've been here to really know what is going on but, this is the polyamory forum. Doesn't that mean your husband and this woman are in love with each other?
You also state this is a non-threatening relationship. Which doesn't preclude jealousy. But then it could be envy as well. I don't know enough about what is going on from this post.
Is he doing things with her that, due to life and things we let slide, he is no longer doing with you? For example, does he snuggle with her watching a movie on the couch and no longer does that with you?
I've been here in the poly relationship we've had. It's a matter of telling him, you know I miss that we don't do X (fill in what X is) any more. I didn't realize this until I saw you doing this with your GF (or he could have told you they did this).
Life gets in the way of our live in relationships and some things go by the wayside without us realizing it.
This may not be the reason you say jealousy is rearing but, you need to get to the bottom of why it is. Otherwise, even if he stops seeing her or just backs off for a while, the issue will not get resolved. It may appear to go away while he isn't involved with her but it really hasn't gone away until you deal with the underlying reason behind this.
Good luck!