This app has no malware content. However, Apple warns of apps from unregistered developers as shown below.
You can just click open using the right button of the mouse.
See more in detail about Apple's security policy:
Fullcovered requires access to the music player to get information about the track from and control the music player. To allow this app to access the music player, you can click OK to the popup window when you launch this app for the first time. You can also set this in the system preferences (System Preferences / Security & Privacy / Privacy / Automation).
Loved and rating: shows different items depending on the music player. Apple Music shows both loved and rating with read/write (Apple Music streaming service only supports loved feature). Spotify shows read-only rating(popularity) only. Hermes shows loved only, with read/write attributes.
Artwork position: the position of the artwork on screen. "L/R Toggle" means that the artwork position changes at every new track.
Get artwork from a third-party database if available: get artwork from a third-party DB when playing Apple Music streaming. This app tries to find the best match by using the track's name, artist or album name. The artwork may not be found or mismatch.
Toggle multiple artworks: In the local library of Apple Music or iTunes, a track can have more than one artwork. If this field is checked, FullCovered shows the artworks in turn at every time interval set by "Toggle interval (sec)".
Show artwork and video title: The album artwork and video title are displayed at the bottom of the window for a few seconds when the video starts.
Autoplay video of your next track: The videos corresponding to your music list are played continuously. The video is the first video from YouTube search for your music.
Back to the video list after playing video: If not checked, YouTube videos related to the video just played will be played continuously.
Search YouTube by: This is a search parameter provided by YouTube.
Sort by priority: Show your preferred video at the top by sorting the videos of YouTube search results again.
Exclude lyrics video: Try to exclude lyrics videos from YouTube search results.
Exclude cover video: Try to exclude cover videos from YouTube search results.
Exclude restricted content: This is a search parameter provided by YouTube.
Alternative play: check this button to play videos in countries where YouTube Music doesn't support.
Max waiting time to video start: Increase this time if "Back to the video list after playing video" is checked and this app returns to your video list directly after playing an ad.
Start a music player with FullCovered: FullCovered invokes the selected music player when launching. If the selected music player is "Auto selection", this app doesn't invoke any player.
Prevent display and computer sleeping while playing: If this setting is on, FullCovered prevents the system from sleeping while a track is playing. If this setting is off, or a player's state is paused, stopped or etc., it follows the system settings of macOS.
In Apple Music streaming: When adding a track by "Play Next" or "Play Later", Apple Music or iTunes doesn't support information of the track.
In Apple Music streaming: Sometimes, it fails to get artwork from the third-party DB when opening this app. It could happen due to a slow network or slow response from the DB server.
When playing a video: If you encounter a message like on the left, check "Alternative play" in the preferences. By checking this option, you can play a video in regions where YouTube Music doesn't support. Some features for YouTube in the preferences may not work properly with this option.