Need help

So my dad had a setup where he had some live rock, about 7 clownfish, and 2 tangs. He decided to add two Green Flowerpot Coral, 2 emerald crabs, and 1 mandarin goby. Before he added these new elements everything was fine, and happy, but the day after he added these new things, today, fish started dropping like flies, he lost both tangs, and he is about to lose about 4 clownfish. He's tested all the levels of the water and the filter is clean, anybody have an idea as to what may be causing this? He said when he put the Coral in it released a white cloud of something, is this a toxin that is now killing fish?



Would really appreciate all advice, and help.