PBT + toadstools don't mix

i thought tangs were reef safe......doesn't look like my PBT has read the rule books.....he has been gradually been knawing his way through it and its looking in a bad way.....don't know why hes taken a liking to doing this....he doesn't even eat it...he just takes a chunk then spits it out on the passing every now and then....he doesn't touch any of the other corals....just goes to show even fish that are supposed to be reef safe don't always turn out to be so safe after all.......heres a pic of the poor toadstool and the perpretrator.

he doesn't even give the other corals a second look catpicklesdog....its just the toadstool he took a liking for....i'll just not be able to keep toadstools by the look of things....i couldn't part with my PBT i was so lucky to get him as any other ones i see on sale tend to be in bad shape....also its not as if hes hungry they get well fed with a varied diet 3 times a day.....i think he maybe likes the texture of the toadstools flesh.