Hi!
AFAIK Yes, they are two separate NMS (but each one is able to call Wireless related features of the other, if I'm not mistaken...so HPE IMC can invoke Aruba Airwave and vice-versa).
Probably HPE IMC is more Data Center centered/tailored so it fits well if you're going to manage DC-class devices/servers of various vendors (Cisco included, just to name it) and it does that so well that it happens that HPE IMC is used as a Trojan Horse to step into a Cisco-centered site...also HPE IMC has a lot o modules (NTA, AMP, etc.).
For what is worth if you have a site deployed with a Aruba Wireless (Controller or Controller-less, it doesn't matter too much) probably Aruba Airwave is just the first NMS one would consider (a first and one stop, eventually)...I mean that it will probably win against the HPE IMC platform + IMC WMS Module solution...without forgetting that Airwave has complete wired related features (especially if your site is all ArubaOS-Switch based...so made of HPE Aruba switches)...OTOH if you haven't (Aruba) Wireless and you're more worried about DC-related features a 60 days free test of HPE IMC 7.3 would be interesting.
Worth to mention that Aruba Airwave 10 (currently Airwave 8 is available) will be around the corner (read about it here).