When we had our Open Brain, we typically fed it at night, because this is a more 'natural' time for them to feed. Often though, their tentacles aren't obvious or visible. Sometimes you can trigger a feeding 'response' by using a turkey baster to squirt liquified seafood, brine, or zooplankton on/near them.
For the most part, we just gently tucked a small piece of raw scallop, clam, or oyster between the 'folds' in the coral. It would almost always quickly consume any offered food.