This is a relic collected on the 3rd map, Dairy Planet.
It is the closest of the two green arrows to follow when you start off the level.
Dairy Planet is unlocked by reaching level 40 on the 2nd map, Inlaid Library.
IMO, it's one of the best to rush for toward the beginning of the game, makes finding strong power ups way easier, early on.
For more info, check out
https://vampire-survivors.fandom.com/wiki/Milky_Way_Map