Montastrea spp. includes M. annularis, the common Star Coral, and M. cavernosa, the large Star Coral which is seen here.  It has some of the largest polyps, nearly 1 cm in size and colonies can grow quite large.  This picture (M. annularis) was taken as the coral was reporducing and the orange balls are bags containing eggs and sperm.  The bags float to the surface where they burst and fertilization takes place.  Most of the corals of this species release at the same time, generally 7-9 days after a full moon, about 2 hours after sunset.  The next day, we commonly get calls reporting "pollution" on the reef! The white substance in the upper right corner is mucus released from the surface of the coral.