Zoas turning Puffy White..dies

[PauChi]

For some unknown explanation the Indo Zoos ( x 4 colonies) that I bought was doing fine until the largest colony piece started to decay, meaning the Zoos started to turn white and dies, and water condition is fine plus the other Indo Zoos are doing fine.
I did the usual FW dip and what would you recommend that I should do as I didin't see any Nudis or weird predators on the Zoos???
Thanks in advance.

Paul