your explaination isn't clear. Let's label the storage units as "internal storage" and "SD card" so we don't get confused.
From your screenshot, it looks like you've configured Spotify to use the SD card for caching music. It looks like you have a 64GB SD card and Spotify is using 5.6 GB. The app itself will always reside on your internal storage. I'm not sure how big it is nowadays, it might be 1GB. I'm not sure why it's showing that it's using 6.5 GB on the internal storage. Have you recently switched the settings for the storage settings?
Can you try the following
1) take note of the capacity reports (as you've done)
2) uninstall Spotify 3) take note of the capacity reports
4) reboot your phone
5) take note of the capacity reports
6) install Spotify (don't play any music)
7) take note of the capacity reports
8) play music for a few hours
9) take note of the capacity reports
Lots of people have been wishing for a "clear cache" button/feature within the app. right now it seems the only way to do that is to remove the app. I do this about once a month because my cache grows a little too big.
Thank you for your answer! I did as you said, but everything is the same. I can find all the files on the SD card and it seems to be there, but in my app manager in my settings it still says spotify uses 6GB. And then my phone starts telling me again about having a full internal storage... I'm so confused...