I would highly recommend doing a lot of research before purchasing more sps.
Many of us spent lots of money before we realized that there are a handful of things that must be known in order to keep sps happy.
Corals, like fish, have a slime coat to protect them. It will also change color when its in your tank. the black spots are where the polyps are located. Also depending on the angle and light hitting it, the color will change.