Hi!! If you're looking for the media player to display multiple sources including Spotify (such as chrome browser etc) unfortunately I don't have a solution, but if you only use Spotify to listen to music (or just want the visualiser to only display Spotify song info I found a pretty quick fix which worked for me.
If you go into the monstercat-visualizer folder and go into @Resources, you should find a file called "variables"

When you open it and scroll down, you should find "Media player settings" - this is basically the code that identifies where it gets the player information from; if you're using WebPlayingNow, for example, you would see PlayerName=Web. Fortunately, Spotify is partially supported by rainmeter in this list: https://docs.rainmeter.net/manual-beta/measures/nowplaying/#playerlist - Only the types Artist, Track and the bangs Play, PlayPause, Stop, Next, and Previous are available, but I found that's fine considering that contains pretty much everything you need for song info. By changing the PlayerName variable to "Spotify" I found it works fine in displaying the Artist and song you're playing. For me the cover doesn't work, and neither does the progress bar, but that hasn't worked for me at all so I'm chalking it up to a problem I haven't solved yet on my end; so it may work for you?

Unfortunately, it won't show anything else you're playing from anywhere else... If you want to listen to other music players I wouldn't recommend this solution... I heard WebPlayingNow with Spicetify has worked for others, so I'd check that out and see if it works (I haven't been able to get it to work, though...)

Hope this helps a bit, I don't mind it being Spotify-specific since I only really listen to Spotify, just thought I'd share what I found helpful~!
Good luck!!!