well there the green encrusting one and the green branching one, both mean and both green hydnophora was big a few yrs or more back then everyone relised how mean they are and well wipe out nice other corals
I think its worth noting that the only documented and photographed cases of them releasing mesentrial filaments occurred when placed too close to another coral I've yet to see any proof of them releasing it randomly into the column to melt your whole tank.
That is my Plating Hydnophora demolishing a soft coral. That coral also killed a Monitpora and a few other sps. Under the right current and the right chemical cues Hydnophora can send out extremely long mesenterial filaments.
Great coral, I especially like the Horn/branch variety, but I do not have enough real estate to give a Hydnophora enough growing space.