Narrowing it down to species is next to impossible without skeletal examination.
I'd say it's a Favia. Maybe a F.speciosa
It looks like a captive raised frag, so it won't look like some of the wild colonies.
Favia speciosa
Characters: Colonies are massive. Corallites are circular and closely compacted in shallow water, more widely spaced in deeper water. Septa are fine, numerous and regular. Paliform lobes are usually poorly developed. Colour: Pale grey, green or brown, usually with calices of contrasting colours. Similar species: Favia pallida and F. truncatus. See also F. helianthoides. Habitat: All reef environments. Abundance: One of the most common faviids, especially in high latitudes.
Source reference: Veron (2000). Taxonomic reference: Veron, Pichon and Wijsman-Best (1977). Identification guides: Veron (1986), Sheppard and Sheppard (1991), Nishihira and Veron (1995), Coles (1996), Carpenter et al
Here's a picture of a captive raised Favia. Probably F.speciosa
It is encrusting down the rock. You can see how it barely produces a skeleton at first as it creeps down the rock onto the white epoxy.