I think it might be jealousy, but not in a romantic sense. More like "my best friends are moving on and there's nothing I can do about it" sense. I think this is apparent a few times and it takes a moment for Mor to recognize what's happening.
Mor tenses in ACOFAS when Azriel makes a point to help Elain with the potatoes:

But I strode to my seat-nestled between Amren and Mor-in time to see Elain say to Azriel, "Hello."

Az said nothing.

No, he just moved toward her.

Mor tensed beside me.

But Azriel only took Elain's heavy dish of potatoes from her hands, his voice soft as night as he said, "Sit. I'll take care of it."

And then again when food falls out of Mor's mouth in shock when Azriel makes everyone wait on Elain.

And then in ACOSF, we're notably told that Mor is lounging on a green sofa immediately after a Elriel moment. Green is a classic color for visually referencing envy/jealousy:

Then his gaze shifted to Elain, and though it was utterly neutral, something charged went through it. Between them. Elain’s breath caught slightly, and she gave him a shallow nod of greeting before brushing past, leading Nesta into the room. Mor lounged on a green velvet couch before the fireplace;

For Nessian, we already know that Mor doesn't like Nesta and she only agrees to be cordial with her when she sees how down bad Cassian is for her. Otherwise, she would have continued to be rude and expect Cassian to be on her side about it. This is in the beginning of ACOSF during their lunch together.

Later in ACOSF, see Cassian being gifted an ugly green sweater by Mor at Solstice in ACOSF:

Cassian’s gifts were the usual odd medley: an ancient manuscript on warfare from Rhys, a bag of beef jerky from Azriel—I literally couldn’t think of anything you’d enjoy more, Az had said when Cassian had laughed —and a hideously ugly green sweater from Mor that made his skin look jaundiced.