These are difficult animals to acclimate. Gaping or flaccid mouth is a sign of unhappiness, maybe even infection. The black stuff can be the remains of a meal, but more likely expelling some of its zooxanthellae. Haddon's should be placed on the sand not on the rocks. I would not feed it until it recovers (if it recovers). I don't personally target feed my anemone s- they grow too fast if I do.