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Looks like a dwindling acan, which most or all are low light corals. Try putting it under an overhang to shelter it from direct light, and in a low to medium flow.
Does it look kinda translucent on the margins, or is that the coloration of the piece? If its translucent, than I think it might be an acan. If it looks opaque and that's the true coloration it might be some sort of favia, prism maybe.
It looks like its doing ok. I hope.. It was a hitchhiker so I cant complain. After looking at my rocks it looks like a purple version of this is growing too.