The Three-Headed Alien Race decided to make a move against the Galaxy Master for three main reasons.
First, the Galaxy Master had just risen to the Ninth Rank—how strong could he be?
Being a Ninth Rank Myriad Layer Realm powerhouse himself, even if the Galaxy Master was exceptionally talented, at most, he wouldn't be able to kill him, feeling entirely capable of retreating unscathed.
The Insect Race Alliance and the Human Alliance were sworn enemies anyway. Whether he attacked the Galaxy Master or not, their grudges wouldn't lessen, and if the Galaxy Master had a chance to kill him, he wouldn't hesitate.
Secondly, this place is the Xuanhuang Secret Realm. The powerhouses who enter here are basically avatars, and the Galaxy Master is definitely no exception.
Since it's an avatar, it won't carry significant treasures with it, and if the Three-Headed Alien Race encountered the real body of the Galaxy Master, they probably wouldn't act rashly.