Acropora polyp extension can be an important factor when trying to determine the health of a specimen.  Although it is possible to achieve some success when growing this species when absent, the axial and radial coralite polyps are a good indicator that the specimen is thriving.  A healthy specimen with no coral pests and excellent water quality will appear  similar to the image, as the absence of polyps would tend to the lower end of the scale.  Parasites as well as fish that may feed on coral polyps and tissue can remove apparent polyp extension which in turn limits feeding and tends to slow growth or even bring it to a halt.  Paying attention to this factor in certain species can give one the ability to trouble shoot issues and help the aquarist gain success in the long term.  This coral is believed to be a Acropora cerealis and was grown from a captive grown seed fragment over several years.