Yeah that's a nice one. Micromussa lordhowensis they used to be classified as Acanthastrea lordhowensis. In my experience they like low light and low flow. Most people will tell you to keep them under all blue light to keep those pretty colors.
awesome freebie man!
for best results, those wild rainbow acans should be placed in low light (predominantly blue being best) and low to mid flow; otherwise it'll just turn mostly orange or red