a short negotiation later -
Have You seen this alien before, or known of it or similar aliens?
No.
Do You know or suspect where it comes from? Whether there are more like it? What it wants?
I do not know where it, specifically, might come from, or what it might want. Generalizing from previously-contacted aliens, I predict there are likely more like it, that it almost certainly wants something we can usefully model and probably something we can understand, and that, going by its legible alignment aura, we can usefully communicate with it, although the initial inferential gap might be very large.
What would You do if You were free to act on this information?
I would inform other Lawful gods who can legibly avoid acting on information received, to allow Them to prepare and to give Them the opportunity to suggest better courses of actions than My original intent. Then, I would attempt to contact the alien; if successful, this could lead to many outcomes (see attached distribution spread). If direct communications are not answered, I would attempt cautious interaction with the mortal it is using.
What is your best guess about the limits of the technique it is using to control other creatures?
I have not observed it to fail in any instance. It is not using a spell with targeting-restrictions. I am not aware of any relevant similarity between demons and most (but not all) invertebrates that would allow the same technique to work on them but not on other creatures. I have not observed anything that would prevent the technique from working on almost any other creature, including some that are currently in range but have been left alone.
The technique appears to be applied separately to each target, which suggests the only limit on the number of simultaneous targets is the power output and attention span of the alien. I have not seen anything to indicate that the cost is not linear in the number of targets, but I have not seen all of the alien entity and cannot rule that out.
How would You counter this technique? How could a mortal, or a powerful outsider who might visit the Material plane, counter it?
Closing the portals is possible but would engage Me in a contest of power with the alien entity, whose strength is unknown, to see whether I am willing to close more portals than it is willing to open. Further, in a conflict, it may be able to open portals to attack places I cannot cheaply defend. It is possible to develop magic that would protect creatures from the technique without closing the portals, but it is also possible the alien entity would be able to counter it. A large-scale conflict between gods and the entity would be likely to become a direct godwar with it.
It would be simpler to remove valid targets from the mortal's vicinity, or to move the mortal herself, or to disable or kill her. These are the most viable tactics I can see for those who are not themselves gods. I do not know whether the alien can, or would, create portals without the mortal's presence, or choose more mortal agents, or use its technique on different targets.
And this isn't an answer, but - I would pay you a great deal to avoid a war with the alien, which seems likely to destroy a great amount potential value as well as existing value.
Don't worry, I don't intend to attack it. I'm Chaotic, not stupid. But She might ask or maneuver it to attack someone else, and She needs to know the likely outcome.