sounds like you got some sort of oil stain there...
can you replace the whole section of durarock with a new one?
even if using glue and stuff would "make it stick" what is the guarantee that it wont separate a couple of month from now...
my lime plaster stuff is almost impossible to chisel off the back of the ceramic tile and literally impossible to get off the basecoat...
i would replace the whole section with the stain without thinking twice.
If that is impossible try to soak/wash it with vinegar, but dont blame me if anything goes wrong...
(vinegar breaks fat (look at those skinny english dudes eating all that chips/fish stuff and mcd/burgerking potato eaters... and also vinegar hardens the plaster)
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