That is a cup coral. It is an LPS from the caribbean or gulf. They glow great under actinics and do not move around or spread quickly. They are non photosynthetic and clear is their normal color. They get white tips when they have been well fed. They have a pretty powerful sting, so keep them away from other corals. I wouldn't worry about your xenia though, it will just grow around them.
I coined the name "invisible sun polyps" for them, but it hasn't caught on.