First of all, are Mopidy-Spotify 4.1.1 of any use to you? Are you able to play anything with it? It depends on libspotify which was intentionally broken by Spotify in May 2022.
Assuming this is still useful to you…
Yes, it is safe to remove /var/cache/mopidy/spotify.
If you delete it, the cache will grow back over time to take whatever size is set in the spotify/cache_size config in Mopidy, which defaults to 8192 MiB, which matches very well with the current cache size you are seeing.
To reduce the maximum cache size to 1GiB, you can add this to your mopidy.conf:
[spotify]
cache_size = 1024
This is documented at GitHub - mopidy/mopidy-spotify: Mopidy extension for playing music from Spotify