What version of Opus are you using? I tried this with the latest beta and didn't have to do anything special to get Nextcloud to show in the Cloud Storage folder - and the icon is correct as well.
I know, I know. The installer of Nextcloud on Windows is far from perfect. But than the registry keys for sync folders isn't that transparent either (for me at least) and the Nextcloud developers keep changing the icons used.
Is there way to find out (in the registry) where the Icons are taken from?