Baihu Yang informed all the God Beasts about Long Chen being possessed before the Black Tortoise King could even speak anything.
He could see that things were looking quite ominous for Long Chen. Even if he killed the White Tiger King when he was possessed by someone else, it wasn't going to matter much for these people since, in any case, Long Chen was a threat to them.
Long Chen had killed one of the God Beasts, and they weren't going to let him live anymore. They couldn't just destroy the spirit inside Long Chen while letting him live.
Moreover, even if they could, they weren't going to make that much effort simply for a stranger like Long Chen.
The Black Tortoise King wanted to save Long Chen without harming him since he was a friend of Zhu Chang, but that was before everyone else came here.