Trachyphyllia sp. (open brain) live in the sediment(they have a conical skeleton to help anchor it down) and IME do best with low/mod lighting and low/mod current.They can be sensitive to some soft corals and recession can occur in the presence of some.Usually,it happens pretty quickly after introduction.Since you have had yours 4 months,that might not be an issue.IME,they need to be fed at least once a wk and the one i have receded quite a bit when i stopped feeding for 2 months.