Kindred's powers aren't clearly defined other then when people are dead/dying they appear and give them a choice to accept or refuse (which more or less comes down to if you wanna go kicking and screaming or a quick but merciful arrow shot). What we do know is that Kindred hate undead very much for the same reason as why Illaoi does where they view undead as an abomination that fell out of the natural cycle of life that needs to be corrected.
Since Morde is undead the Kindred most certainly don't approve of his existence but as to why they don't go over to the Shadow Isles and commit a purge that'd make Lucian's crusade pale in comparison is unknown because again we don't really know the limits of their power other then they mark those soon to die and show up when they do.
I will say they seem to be the personification of death but aren't the reason why its caused (I.E. they don't make the people die)
But getting back on track Morde, like Need Gold 4 Tent said, is a special case even among undead because for one didn't need to be effected by Shadow Isles magic to become undead and two, pulled a Doomslayer 'too angry to die' move and came back not once but twice like and absolute UNIT of a man.
Its implied that as long as he manages to kill enough people when he reforms he can resurrect himself indefinitely so the only way possible to really put him down for good is if they stopped him the second he reformed (but again its entirely possible that he could just convince more mages to bring him back like his first resurrection) so the most sure fire way for Kindred to put a stop to him is if they locked him away in some sort of personal hell where no one could speak to him and vise versa or destroy his soul which I don't know is possible but something I doubt the Kindred would approve of.
But just in case I missed some of the questions in my analysis I'll answer them best I can right now and fill in as a TL;DR
Does kindred hold power over morde?
Probably no more then any other undead/living being since she doesn't seem to actively hunt down undead and appears to be content to wait until something re-kills them.
Or is he just some external anomaly she doesn't know how to deal with?
I mentioned it before but he is a special case where he came back through unlikely means and seems to be able to repeat this as long as souls are in abundance in whatever realm he goes to when he falls though it is entirely possible Kindred would have to take more drastic measures to ensure he stays there.
Does she have a plan for him eventually?
Kindred has a plan for everyone to die eventually but what do you do with someone who's repeatedly come back from the dead. It doesn't help Kindred is so vague and mystic like we can't really get a bead on their agency because if they are trying to do something they are are taking their sweet time doing it but its entirely possible they are waiting to see if a mortal/force put him in his place before they resort to more extreme measures.