I recently contact Aqadeoco about this rock and they respond this:
The coral tuff should be save for usage in your reef tank.
As all natural materials it should be washed and cleaned and cured before usage.
The reddish color should not be a problem.
It comes from the soil the coral tuff was taken from.
Some soils are reddish others more yellow.
Anyway, using natural materials in tanks is never 100% save.
It can always be the one of a million rocks with some toxic elements inside.
But what we can say is, that we sold many tons over years of the coral tuff.
It is used in small reef tank up to public aquariums without any complaints.
Means to us: it is save.
Besides: Even better might be our Jurassic Reef Rock ( Flyer attached ).
This type is really well cleaned and has a perfect red algae look.