This could be a species of Porites. Porites is a common SPS (SPS - acronym for Small Polyped Scleractinia, sometimes called Small Polyped Stoney, refers to such genera as Acropora, Montipora, Pocillopora, Porites, Seriatopora, Stylophora and others) coral on Indo-Pacific and Caribbean reefs. It can be encrusting, finger shaped, or dome shaped. It has tiny polyps and sometimes commensal fanworms (Spirobranchus giganteus). Porites usually requires strong illumination and wave action as they are most abundantly found in areas of high wave action and tidal flows. Similar to Montipora, Porites can be distinguished by much larger corallites (polyp walls) that are filled with septa ("teeth"). Some really cool corals are often found hitchhiking on live rock but they are very difficult to describe sometimes. Some encrusting hard corals on live rock look much like coralline algaes with the exception that their surface is rough or fuzzy looking as opposed to the slick and smooth appearance of coralline algae. A good book for beginners, The Practical Guide to Corals for the Reef Aquarium by Eric Borneman and Ed Puterburgh, is a excellent, inexpensive book to consult for the identification of corals.