From the image you posted it looks well like the coralites are independent and tubular or pentagonal or hexagonal tubes...could be in fact galaxea...looks like one.
Have you ever seen if it extends long polyps?
Siderastrea you say...hmmm:scratchch....well it doesn't look like a Siderastrea. Have a look here. It seems to be a Galaxea coral. If it reallyt is, then it is a very aggressive coral.