We've never really paid attention to this icon before but from having a look at some of the Hikvision Support documents for Hik-Connect it looks like this is a storage indicator.
If you have just an IP camera with a MicroSD card fitted you would see a little blue MicroSD card and when you have an NVR connected you see this little storage /HDD symbol, I would also assume that blue means the HDD is formatted/enabled and red probably means the HDD is either unformatted, disabled, or faulty.
There is some information in the Help section in the Hik-Connect app but it doesn't specifically state what these icons mean, my above explanation was just a logical guess based on the vague information Hikvision provided.
Are both NVRs configured to record? If so, have you checked playback in the app for both NVRs to make sure there is no recording/playback issue with the NVR displaying the red icon?
Below is a screenshot of some information about these icons from the Hik-Connect release notes:
It mentions the icons relating to "frequent operations and status information" so it could be that they just signify that you have recently viewed continuous footage on one NVR (Blue) and smart event footage on the other NVR (Red).