to me this looks like an acropora tenuis. needs some coloring up but with the tips looking like they do thats my best guess and in id'ing acropora thats about all you can do..guess.
The growth form/color doesn't look typical for A. tenuis but it certainly could be. Also A. tenuis doesn't typically show that much polyp extension except at night.
Many of the Acropora corals that only extend polyps at night in the wild change in the home aquarium and begin to polyp during the day too. One of the theories is that we tend to refrain from keeping polyp nippers with them while in the wild they would occasionally be nipped during the day by fish.
Kevin, I have about 4 color variations of the tenuis and every one extends polyps day and night. The pics I've seen on another forum always show them with the fluffy looking polyps. That was what I was going by with this pic plus the tips. But like you said these are subject to being in captive home aquaria and for mine for several generations.
The tips do look very typical but the coral growth form is slightly different. This could be due to the size of the coral (it just hasn't grown into a colony yet) or due to flow. I've seen some branching type large stags turn into table growth forms from exposure to high water flow. Once the flow was removed it went back to the staghorn growth form. This is one of the reasons it's so hard to come up with a species ID for Acropora, as it can change dramatically in the home aquarium.
to me this doesn't look like the A. plana that I have. Or most of the pics I've seen. Then again it is probably like Kevin says. This coral still looks a lot like my 2 tenuis colonies. I got another one yesterday and the guy it came from had the one I got plus 1 of the same coral in his reef. The 2 were completely different. One had been in his reef tank and mine had been in his prop. tank. The lighting was exact but the flow was completely different.