Are they under high light and is the flow moving in that direction?
I have a bunch like that (mine are small types and only grow to be about 3-4" across). They'll grow with the current rather then the light, if the lighting is more then enough for them. Mine have been steadily 'walking' (as the new stem growth anchors the old growth dies off) across the rock they started on, until they hit the edge and now at growing horizontally away from it.