Yes, your piece looks like Micromussa diminuta. Can't say for sure if it is or not.
M. diminuta
Characters: Colonies are massive and cerioid, with pentagonal or hexagonal corallites 3-4 millimetres diameter. Walls are thin. Septa are beaded and columellae are poorly developed. Colonies do not have thick fleshy tissue over the skeleton. Colour: Brown, with pale grey centres. The surface is usually speckled white. Similar species: Micromussa minuta, which has larger, more rounded corallites with fewer, thicker septa. Underwater this species can readily be mistaken for a faviid with small corallites. Habitat: Shallow reef environments. Abundance: Uncommon.
Taxonomic note: See ‘New species described in Corals of the World’ (Veron, in preparation) for further information. Source reference: Veron (2000).
Remember that these pics are taken in the Ocean, and not under Actinic lighting.