Have you loaded a new cert onto TrueNAS?
If the android Moon+ App is not going to the TrueNAS WebUI, then I don't see how the cert is the issue?
Has anyone else used Moon+?
Yes, I loaded the 'calibre-web' certification I created to be used for the calibre-web application. I then used truecharts ingress function in conjunction with the certification to enable https connections. Moon+ is simply the interface used to access the calibre-web instance.
I will point out, I use this same set up for all ofy applications. Nextcloud, calibre, calibre-web. Ombi, booksonic, airsonic... Everything that isn't internal network only. The only problem is with the interplay of moon+ and calibre-web, though, I sort of think it has anything to do with my certs or setup. I just received an email from the dev stating that they tried building the app to bypass Google's trust anchor and the build got kicked off of the play store... SO, very unlikely it's not the apps fault alone.
That being said, there are other readers that can essentially perform the same, just not in as nice a package as moon (it's a pretty badass reader... I just refuse to not use SSL, as standard http works perfectly fine on it)