Usually, millepora is fatter and prostrata is narrow/skinnier with a more pointed tip. A. microclados will have elongated/tubular corallites that are spaced out and not on top of each other. I've seen A. prostrata not listed as a species, like A. spathulata. The difference between spaths and milli's is girth, spaths are bigger/fatter and they have a more noticeable lip to the corallite edge and they seem to lay flat or have a more open look to them, where milli's and prostrata don't. But I've seen milli's and prostrata share those characteristics too, so who knows?