I'm guessing they're asking what the difference between A. millepora and A. tenuis is.
Let's start off with the similarities. Both grow in a corymbose shape. That's about it.
Acropora millepora have slightly wider branches and more tightly packed radial corallites with thinner walls. For coloration, A. millepora are usually bright pink, red, or yellow. The axial corallites are sometimes colored yellow. Towards the inner-colony, the polyps are extended. Similar species are Acropora spathulata, which has thicker branches with more flared/scale-like corallites.
Acropora tenuis have thinner branches and less crowded radial corallites with thicker, slightly flared walls. A. tenuis are usually pale or cream in color with highlights on their corallite tips.