Palythoa Grandis. Technically they were once characterized as protopalythoa grandis but I don't know if that is still the case today. I have these in my tank. The coloring is drab but the mottled pattern is unique. They reside in the sand bed and grow in very low light/current. Perfect for filling in spots at the bottom beneath other corals. They eat fish food so require no special attention. One of my favorite drab/filler corals. Some Pacific Ocean versions come with some green mixed in the white. I prefer the Caribbean straight brown/white.
They used to go by the name moon polyps.
Cinnamon polyps are (or used to be) a different brown polyp that was co-classified also a palythoa species but I have not seen these in the hobby lately. The oral disc was wider but the tentacles were longer.