Creator here: I don't know if they have a name but I based the design of that interaction off of Propellerhead's Thor iOS app which is the only other example of that overlay that I could find.
That's not in Thor on (desktop) Reason is it? I don't remember seeing it, but it's a really cool feature imo. I always have trouble fine tuning knobs, and I haven't figured out how to map them to my midi controller (the preset that it said would work didn't work. I can, at most, get one slider to map to a random knob). The pop up slider seems much easier to use with a mouse, even if the motions are relatively the same.