Yup, I have mine in the bottom of the tank, under a little rocky overhang. It stays mostly shut during the day, but starts to open around dusk (I have LEDs set to gradually power up/down on a parabolic setting like the sun). THe few times I've peeked at it in the middle of the night it has been huge -- about twice as big as it was in the picture above. The flow down there is kinda low, but I think they will tolerate moderate flow. I have a Dendrophyllia in the same tank that does fine with the moderate flow.
To encourage yours to open I would wait until lights off or just before that and kill your pumps. Squirt oyster eggs or the thawed "juice" from a frozen cube of food around your Balanphyllia to encourage it to open. Then once the tentacles are open you can give it a mysis or brine shrimp. There is a good post here about the "Tupperware method" of teaching your NPS coral to open up for feedings.
Thanks for replying. I've moved mine to the bottom under a ledge and will attempt to spot feed at night when it's open the most.