Those acropora do pretty well with less light than most acropora do. So you might just be flying with the lighting that you have. Make sure that you acclimate them to your lights very slowly. You'll find these corals are a lot more sensitive to light shock than most other acropora are.
I have one directly under a 400W MH. 6" Below water surface with bulb 8" above water surface. Got it about a month ago. It's a little bigger than yours. I'm just now starting to notice some growth and new branching. Mine seems to be doing well where it's at. I don't have a clue how fast they grow. Anyone have an idea how fast they normally grow??