The Loops APK distributed via loopsusercontent.com are safe!
I've seen a few reports about it being flagged as malware, and it's a false positive.
You can see for yourself on VirusTotal!
https://www.virustotal.com/gui/file/faef6780d21169fa14fb9e9aebfe6678bf8a18ae27b7ca9a34102934dabb8816
Don't worry though, the source code will be published, and the app will be available on the Play Store/App Store + F-Droid.
The sideloadable APKs are only temporary!